Whats the model/type of computer you have, and whats your Operating System? (Windows XP/Vista/etc?)
Do you have a Windows disk (can also be considered a "Recovery Disk" if your computer is a prebuilt model like Dell, HP, etc.)?
You can perhaps do a repair install on your hard drive. Repair installs on Windows installations don't delete anything, they just fix up whatever's corrupt/missing in your system folders, keeping your valuable documents, settings, programs, etc. intact. I do it whenever one of my system files go awry making my system not boot properly. Works like a charm, and all my documents and such are still there.