yyahoo: Boot your computer from your OS disc (whichever version it is) and select the repair installation option. It should correct a damaged registry, though nothing is guaranteed. It's better to try that first before a completely fresh install.
Elmofongo: How do I boot my computer from my OS disc? And also is it possible to restore a registry that was deleted?
When your computer boots up, there is an opportunity to hit either the delete or F2 key (varies depending on the bios/age of computer) that will take you to the bios settings. In the bios settings there is a section to change the boot drive order, essentially the order that your computer will look on each drive (whatever kind of drive it is). The first drive that it finds that is bootable, it will boot to. In the bios, you will need to change the boot order to make your optical drive first. Then, make sure your OS disc is in the drive, save the bios changes and reboot. After booting off of the OS disc, you will be given options eventually to install the OS from scratch or to run a repair installation. After the repair has occurred, you will want to go back to the bios setting and change the first bootable drive choice back to your primary drive.
Registry *entries* can possibly be restored, but if the entire registry file is lost... Well, that *is* what defines windows. If it's gone, you might as well install from scratch...