My first recommendation (until I have a bit more information) is to run DRWatson. You can do this by clicking on the Start Button, and Selecting Run... Type drwatson.exe in and hit <enter>. Go into it's preferences and select it to load on startup.
If you double click on it in your systray (by your clock) it will do a scan of your computer. The nice thing about DRWatson is, it gives you more 'details' about an error, and it logs the errors for you. SO, after you start up again, you may be able to check that log for the problem.
If this gives you more information, post it here. It may help others to help you with the problem.
Patrick.