I'm not sure why you want to uninstall .NET Framework 3.0 - we instead need to make certain it IS installed and enabled and that .xps is also enabled.
Here's how to make sure you have .NET Framework 3.0 installed and XPS Viewer enabled: What is the XPS Viewer?
. The article suggests if .NET Framework 3.0 isn't installed (and I get the impression that might be the case from your post though I'm not sure how or why that happened), to install it using the download from the link in the article but the download says it isn't for Vista and I cannot understand how the two articles both from Microsoft can present such diametrically opposite guidelines - so to be on the safe side I wouldn't do it yet (but we may come back to this later if all else fails).
In the meantime, let's try some general troubleshooting, starting with a check of your system files and hard drive for corruption or problems. Go to Start / All Programs / Accessories / Command prompt and right click on command prompt and click run as Administrator. 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. If that doesn't work, then please post any error messages from the Event Viewer concerning xml viewer or net framework (Start / Control Panel / Administrative Tools / Event Viewer) and there WILL be some if not quite a few or even many. If you have a hard time finding any, then do whatever causes the error several times and then recheck Event Viewer sorted by date/time and the errors should be near the top. Click on the errors to get more details and copy those details here. If the errors are all the same, one copy will do. If there are different errors, then try to get one of each or, if too many, at least a good sampling. Here's how to use Event Viewer: http://www.petri.co.il/vista-event-viewer.htm. Look in the System section and the Application section. 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.
If that doesn't work, we'll need to do a system repair/upgrade using a genuine Windows Vista Installation Disk (one you own or one you can borrow from ANYONE) - recovery disks or OEM installation disks will not do the trick. To do this you have to also be able to boot normally - but that doesn't seem to be a problem here. 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 hope this helps. If not, post back with details of what happened when you tried these options (as well as the requested results, attached files, copies of reports, and any other details you discover that you think might be useful for us to know) and we'll see where to go from there.