I think I had a similar problem with the comp of one of my friends. It wasn't blaster since the OS was Windows98. I think it might have been some software incompatibility. Rather than searching for the problem, since no viruses were found, nor spywares, I reinstalled a new version of Windows.
If you can't fix it, reinstall windows, after saving your necessary data, and then make a backup (with ghost) of the OS. There on after, wherever something that pisses you off appears, you can use that backup and have a new OS in less than 5 minutes.
It doesn't worth the time to try to fix whatever problem you may have. If it takes you more than 2 hours to fix the problem then better do what I said.
Just a question, do you have any codecs installed? I had a similar problem with one of the codecs I had. My problems was that whenever I opened WMP or any other movie viewer, everything on my desktop closed (but comp did not reboot).
EDIT: If you use WinXP try using the restore option. Choose a date on which your computer still worked. WARNING: This screws any apps and sometimes files you might have saved/installed after the restore point was made.