I had a computer that did this like 10 years ago, and it turned out to be a fatal virus. Not saying thats whats going on here, but if i were you i'd try to boot in safe, and then run your virus software in safe mode. Good luck.
Try to reinstall windows.. if you need to do a backup, take teh drive out and put it in another comp or a external enclosure and use another comp for the backup. If reinstall doesnt work, hardware of some type. Hard drive would be my guess.