I have not made any changes to the hardware recently for this to cause a problem, but have installed TMpegEnc, and am not sure if this is causing the problem as I am not able to go into safe mode and uninstall this application. I am also running a up-to date Antivirus software so I don't think it is a virus causing this, but I may be wrong.
If Windows XP encounters a "blue screen" condition during startup, it reboots automatically instead of displaying the information. If you can't get to your PC to boot in Safe Mode, you made need to try a repair installation. Just boot from the Windows XP boot disks, or insert the CD-ROM and choose the repair option from the installation screen.