Sounds like corruption, most likely due to a virus or bad sector on your hard disk (leaning toward virus). Try booting in to safe mode by pressing F8 right after the system POST, and selecting Safe-Mode. You can also try Last Known Good from that same menu.
All else fails, you may have to rebuilt the OS using your Windows or manufacturer's disk. If you need to backup your data, take the hdd out of your machine and slave it to a system with a compatible IO controller. Good luck!