boot with CD or USB, copy the file over from other PC

My buddy's Dell once had the same problem. I booted his PC with WinPE on a USB stick, copied the corrupted file from a good PC and everything went OK.

There are WinPE disc images you can download. You can burn a CD if you don't want to boot with USB.

Or you can restore the OS from the recovery partition. The newer PCs come with a recovery partition instead of OS CDs. Hopefully you didn't delete the recovery partition.

Or you can take the HDD out, put in another PC and overwrite the corruptted file.