Originally Posted by Blameless
It's perfectly possible for an unstable OC to corrupt a Windows install beyond repair.
Upon boot, and during use, things are constantly being written to the drive(s). If you have an unstable OC bad information can be stored, permanently (well untill it's reinstalled) disrupting the function of your install. Unstable memory subsystems are a prime cause of this, but highly flaky CPU OCs can do it too.
I try not to ever boot into windows on a system that cannot pass a few loops of Memtest86+. While far from ensuring complete stability, this is normally sufficent to get into windows without corrupting anything so more complete testing can be done.
If clearing the CMOS does not help, it's time for a reformat. Also, never install windows or do a bios flash when OCed, unless you are 110% that your over clock is absolutely and completely stable.
^ What he said.
Best thing to at least try out, before reinstalling your OS, is reset your CMOS. Unplug your PC first, then find the CMOS jumper and switch it to the "reset position" for 10-15 seconds, then switch it back. Then start your PC up again and enter your BIOS and set anything needed (like boot priority, SATA or IDE config, etc.) and try booting back into Windows.