Welcome to Vista Forums! Please don't be intimidated by the length of this post
. I could have broken it into many separate posts with a few procedures in each, but I think it helps to have it all available so you can move at your own pace without needing to post back and forth. Many of the items do not take much time or can be started and you can do other things while they work. It's also possible some won't be required if the issue is identified and resolved by one of the earlier procedures (not likely in your case, but still possible). I urge you to try them all (even if tried before - like SFC but this time using the disk instead of safe mode) and to use the order presented (because some of the later ones depend a bit on having tried some of the earlier ones that may effect some repairs even if not solve the problem entirely) - though you can jump around a bit if you feel some option sounds more likely or you learn something that makes some later option a better choice to try next. But please do try them all.
If you find this too overwhelming and don't want to dedicate this much effort, go to the section after the starred line and just do those two things. While it won't be as effective and may miss the solution, at least we can try to make some progress on the upgrade and ignore trying to fix anything else. Otherwise, read on. Do you know when this problem began? Try a System Restore to a point in time BEFORE the problem began. Here's the procedure: http://www.vistax64.com/tutorials/76905-system-restore-how.html. Be sure to check the box to show more than 5 days of restore points. If the first attempt fails, then try an earlier point or two. NOTE: You will have to re-install any software and updates you installed between now and the restore point, but you can use Windows Update for the updates. Use the recovery disk if the system prompt doesn’t work. The recovery disk works a bit different from the above procedures but if you follow the prompts from the System Restore menu option with the above information you should be able to restore with no problems. If the System Restore doesn't work, do a Startup Repair by booting to the genuine Windows Vista Installation Disk (or one you can borrow from ANYONE) or from a Recovery Disk with recovery options included on it. Here's the procedure: http://www.vistax64.com/tutorials/91467-startup-repair.html. To boot to the CD you may need to change the BIOS to make the CD-drive first in the boot sequence. To do that, wait for the screen that tells you the F key to push to access the boot menu or boot setup. Push it quickly. Make the changes, save your work, and exit. Put the CD in the drive and reboot. When prompted, push any key to boot from the CD. If you don't have either disk, you can make a bootable Recovery Disk using http://neosmart.net/blog/2008/windows-vista-recovery-disc-download/ along with burning software like: http://www.snapfiles.com/get/active-isoburner.html and, of course, a blank CD (perhaps made on a different computer or perhaps in safe mode with networking if that is working). If that doesn't work, then let's check some of your system files: Go to Start / All Programs / Accessories / Command prompt and right click on command prompt and click run as Administrator (you can skip this step and just go to Command Prompt if using the disk). If using the disk, cd to C:\Windows\System32. Type sfc /scannow and enter and let it run. It will scan and try to fix some of your system files. Hopefully it will complete with no corruption it could not repair (if there is such corruption post back here or try to analyze it to find the problem file(s) using http://support.microsoft.com/kb/928228. Try to attach the report (you may need to copy it to your desktop before it will permit you to attach it) here so we can see if they can be repaired with good copies from the installation disk (unless there are too many). While in Command Prompt, type chkdsk /f /r and enter and let it run. It will want to schedule itself to run at the next restart. Answer yes and then reboot to run the program. It will scan and try to fix any corruption or bad sectors on your hard drive and mostly remove that as a potential cause. Considering the issues you're having with disk space when there shouldn't be any, perhaps it would help to defragment the drive. Here's the procedure: How to Defrag Your Hard Drive in Windows Vista | eHow.com. If that doesn't work, then please post any error messages from the Event Viewer concerning startup (Start / Control Panel / Administrative Tools / Event Viewer). Here's how to use Event Viewer: http://www.petri.co.il/vista-event-viewer.htm. Look in the System and Application sections. You may be infected with malware. Try to run anti-malware programs (in safe mode with networking if necessary). To fix this problem (if it is a problem) download, install, and run the following two programs: http://www.malwarebytes.org/mbam.php and http://www.safer-networking.org/en/index.html. You may also want to try the new, free Microsoft Security Essentials http://www.softpedia.com/progDownload/Microsoft-Security-Essentials-Download-131683.html (with the caveat that only ONE AV program can be installed and running on your system at any one time). Use removal tools when appropriate http://www.raymond.cc/blog/archives/2009/05/05/comprehensive-list-of-uninstallers-or-removal-tools-for-antivirus-software/. You may also want to try the free Avira at: http://www.free-av.com/ and Avast at: http://www.avast.com/index. Reboot after completing all the scans. You may also want to try the free OneCare at http://onecare.live.com/site/en-us/center/whatsnew.htm and let it run all the options (except the registry cleaner) because that’s good maintenance (it will take some time to complete but can be done in the background). If that doesn't work, try a clean boot http://support.microsoft.com/kb/929135. If the problem goes away then it's just a matter of tracking down the culprit causing the problem. Follow the procedures in the article. Once found, delete, remove, deactivate, or uninstall it. Once done be sure to reset Vista back to normal status as explained in the procedures. If the problem occurs in clean mode then just restore the system to normal status and reboot - this solution is not going to work. Boot into Safe mode with networking and see if the problem occurs there. Either result helps us narrow down the search for the cause. You may be having troubles with your device drivers, firmware, and/or software. Go to Device Manager by going to start / search box and type device manager and enter and then double-click on the program icon that appears. Check each device for a red x, yellow ! or white ?. These identify devices with problems (probably drivers, but also conflicts or something else). Click on each for further details and troubleshooting tips. If you need to get drivers, do so from the computer vendor or device manufacturer (NOT from Microsoft Updates). Proceed as follows to get the drivers: http://pcsupport.about.com/od/driverssupport/ht/driverdlmfgr.htm. Once you have the drivers, you can install them through Device Manager as follows: http://www.vistax64.com/tutorials/193584-device-manager-install-driver.html. You may also want to try the Driver Verifier Manager to see if you can identify the driver or device responsible (assuming that is the cause): http://support.microsoft.com/kb/244617. Let's test your hard drive and RAM for hardware problems: To test your hard drive, try HD Diagnostic (http://www.carrona.org/hddiag.html) or this test: Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp (http://www.tacktech.com/display.cfm?ttid=287). If it fails the tests, replace it. If it passes the tests, then there's probably nothing wrong with it. You can also test your RAM at http://www.memtest.org/. Here are additional instructions that may help: http://www.sevenforums.com/tutorials/105647-ram-test-memtest86.html. If it fails, test each chip individually to find those that are bad and replace them. If it passes, RAM is probably OK. If that doesn't work, we'll need to do a system repair/upgrade using the genuine Windows Vista Installation Disk (one you own or one you can borrow from ANYONE). To do this you have to also be able to boot normally. Here's the procedure: http://www.vistax64.com/tutorials/88236-repair-install-vista.html. Although this will not affect your data, settings, or programs, you should still backup your data before starting just to be on the safe side. You may have a lot of updates to re-install (including any service packs you had to remove). If the version on the system came with SP1 or SP2 pre-installed and the disk is an earlier version, then you'll need to make a slipstream disk as follows: http://www.vistax64.com/tutorials/151606-vista-sp1-slipstream-installation-dvd.html. I'm hoping that the previously performed procedures will resolve the insufficient disk space error message you've been getting. ************************************************** If that doesn't work, go to Blue Screen of Death (BSOD) Posting Instructions and follow the instructions. Although this is for a different type of problem, the information is still valuable for diagnosis and may help us identify the problem. Also, if you still have the "no space on drive C" problem if you again try to upgrade with the disk, then please install WinDirStat WinDirStat - Windows Directory Statistics which may help identify why the space is not matching what seems to be available since it gives a better analysis of how all of the space is used on the drive than anything Vista provides. Please let us know what it finds if there's anything of interest. Hopefully, one of these procedures will identify and/or resolve the problems. If not, post back and we'll try something else. I have a few more ideas if you continue to experience the same problems and still can't upgrade due to the disk space error, but this is enough for now. Your feedback on what happens at each step and concerning the results of some of the procedures (and the requested files) will help in defining what we try next and in what order we try it. Good luck!