Corrupted Hard drive maybe from a power surge or unplug while the hard disk was running full throttle? During start up, can you go into repair computer.. or whatever it's called? Factory reinstall should be an option in there. Vista computers now hold a partition on your drive that is a copy of your whole operating system like it was when you took it out of the box, you shouldn't need a backup if you are running vista... just restart it. When it shows the windows insignia turn the power off. Then turn it back on, you should get the "Windows had an error" screen and the repair options... pick repair computer... when you get in there do a factory install, and wait about 20-30 minutes and it should be back to new...