Welcome again to Vista Forums!
Please check out the following tutorial and see if it helps (it almost always does - though which method works varies from case to case): Power Options and Sleep Mode Problems
I hope this helps. If not, post back and we'll try to see what else we can do to track down and eliminate the cause.
In the meantime, if your computer "takes forever to startup" (a different problem, but since you mentioned it I thought I'd address it), I recommend a little cleaning and maintenance as follows:
Here's my little blurb on improving space and speed in Vista systems (and it also suggests other things of value that you can consider or ignore - the choice is entirely yours). All of this is your choice, actually - I'm just presenting some options that may help and that have helped many others (and that I pretty much use myself though sometimes I do procrastinate I'm sad to admit and there are some things I won't do no matter how beneficial - for example, I refuse to delete my cookies (but that's a personal choice as I prefer the benefits of having them when going to previously-visited web sites over the limited amount of space recovered and speed gained given their small size)). Much of this should be part of normal maintenance, but that includes some other things (like creating periodic system restore points - probably at least weekly as they don't necessarily improve speed, space or performance and may in fact even do the opposite in some cases) not addressed here.
A great many things influence the speed and performance and available space of your system - mostly the existence of various files and what's loaded on the system. There are ways to handle this in Vista (and you should follow all the suggestions) - and of course the amount and type of RAM and the speed of your processor and motherboard (but I'm just going to discuss software solutions here - you can go to your computer vendor/manufacturer for hardware solutions if any are available (I, for example, am maxed out at 2GB RAM by my motherboard and there is no better motherboard available for my model of laptop but if your system can handle it, Vista 32-bit can use up to 4GB of RAM (well, almost 4GB, more like 3.5 actually) and the 64-bit version can use even more). If space is the issue, replace your hard drive with a larger one, add a second hard drive (if it is a desktop system), or add an external hard drive. Those are hardware answers (which cost money). We'll now discuss software solutions (which are mostly if not entirely free - at least the options presented here - there are 3rd party programs that provide additional options but they really aren't necessary).
One very important thing to begin – do NOT use any Registry Cleaner or Registry Booster or Enhancer, or Repairer or Fixer software (or whatever it calls itself) no matter what it says (scans will tell you have thousands of errors just to get you to download and install the program or worse, buy it) or where you got it (even safe sites sell these products but they are not worth the headaches they can cause). Most do nothing of value and some can do great harm (to the point where you need to do a clean install to fix the problem – and that’s not even counting the fact that many such programs are actually malware in disguise). Vista keeps the registry clean on its own quite well and doesn’t need any help.
To improve your speed and space, do Disk Cleanup using: http://www.vistax64.com/tutorials/76073-disk-cleanup.html. Delete as much as you feel comfortable doing – nothing deleted there will harm your system in any way. This includes deleting the Temporary Internet Files. If you want to keep this from building, go to Tools / Internet Options / Advanced and check the box Empty Temporary Internet Files Folder whenever browser is closed and then that folder will always stay empty (except when you're using the program).
Also do an Optimization: http://windows.microsoft.com/en-US/windows-vista/Optimize-Windows-Vista-for-better-performance. Do all of the options (or at the very least, consider them - many can help a great deal).
At least once every 2 months or so, run CCleaner http://www.piriform.com/ with as many options as you choose to delete (but be careful because this program if not used properly and with caution and with you paying attention to what you are doing can delete important and even critical files that could cause a great deal of trouble).
System Restore can take up to 15% of your partition's space but that can be reduced. It's a trade-off. The less space you allocate, the fewer restore points you will be able to retain - but many people reduce this to save some space while leaving enough to save 3-4 restore points at least (which is usually enough - until you need them). Here's the procedure: http://www.vistax64.com/tutorials/76227-system-restore-disk-space.html.
Go to Start / Search options and type in msconfig and enter and then double click on the program icon that appears. Go to the startup tab and uncheck any program that you don't need starting at startup. That will probably be the majority of items there - if not most of them (some are needed like the AV program and Windows Defender but most are there to make opening the source programs faster and make you think they're more efficient). This will free up a lot of RAM and help a lot in making your system faster (though perhaps not so much that you'll be able to notice the difference - but maybe depending on how much unnecessary stuff is loading at startup). When I did this on my system I removed over 90% of the entries and suffered no ill effects – to the contrary, I noticed startup was quicker, response times when working were better and I could open more programs at the same time and still have no problems, and shutdown was also faster. Better still, download Autoruns http://technet.microsoft.com/en-us/sysinternals/bb963902.aspx which is much superior to msconfig and click on the Everything tab and do what was suggested above (though here you can’t delete everything as with the start menu in msconfig because there are some very important files and services included which MUST not be deleted or it can cause serious harm to the system). If unsure, leave it alone.
You can accomplish some of these tasks (and more that aren't entirely related) by using http://onecare.live.com/site/en-us/center/whatsnew.htm (which also searches for malware). I do this on a monthly basis just as a part of normal maintenance and I suggest you do the same (except for the registry cleaner – no not even Microsoft’s own product is entirely safe and the product has been removed from the paying marked for reasons unknown though it’s still available here for now but probably not for long). Make sure you select a full scan - it will take a few hours (mine can take around 5-6) but can work in the background so start it when it has enough time to complete.
You also want to have good AV and AS programs on your system (so no malware is operating in the background draining resources). Windows Defender is a good AS program. The new, free Microsoft Security Essentials http://www.microsoft.com/security_essentials/default.aspx is an excellent AV/AM program that has one of the least impacts on system performance and resources according to reliable sources and it does the job very well. If you're using MSE, you don't really need Defender and MSE will deactivate it when installed (but you would with many other AV programs - though some of them also deactivate Defender). I would also suggest downloading http://www.malwarebytes.org/mbam.php and running it monthly to catch anything that MSE might miss.
If you follow this advice and do it periodically (at least monthly) then I believe you will have as fast and clean a machine as it is capable of going given the hardware.