What is your current screensaver? It could be a bad screensaver if it was installed from the web.
Wait untill somebody verifies this next step before trying it:
(Based on XP)
Find all screensaver files. They have the extension .scr then delete the any that are 'third-pary' i.e. not part of the OS.
I tend not to use screensavers from the web as they often contain 'crap' such as spyware/ad-aware etc
DON'T DO THIS UNTILL ANOTHER FORUM MEMBER SAYS MY MEMORY SERVES CORRECT!