Does it give you command prompt ? If so insert installation CD and type: sfc /scannow.
Otherwise, you can delete Windows directory and install fresh (Leave file system intact), so that you can recover settings and data. Your settings would be in Documents and Settings\<username>, your data should be intact. All you have to do is re-register your software in windows, but most cases re-install software to register it. Overwrite old data might give you the Repair option which is faster and keep settings.