Probably some malware infection. If you have anti-virus/malware software installed, try to run it in Safe Mode. Or try booting in Safe Mode with network support and run some online scanner like FSecure at http://www.f-secure.com/en_EMEA/sec...
Do you get a menu option to run it in safe mode or does it just go to safe mode automatically? If you get the menu, try last known good configuration. If you have an xp disk, do an xp repair. If that doesn't work, it's a hardware problem.
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