Windows XP will do this when the computer is not shut down correctly, or when a new program or file has been installed and has corrupted or damaged files. Next time it happens, let in run in safe mode, when it produces pop-up to ask if you want to continue to run in safe mode, answer yes. Log on and check through your add or remove programs on the control panel. If all looks fine here, (no strange or unfamilliar additions), close and restart. If you should find a program that is unfamiliar, research it through windows explorer. Spyware tends to do this to windows xp operating system. If you see listings such as: My Search Bar, New Dot Net, Wild Tangent, or listings of the like, remove and restart.