Originally Posted by Xaccers

You're using an illegitimate version of the software then.
That's the problem.

If your original CD has messed up, contact MS UK 0870 6010100 and talk to them about getting it replaced, normally around £11 for media sets though that was a few years ago.

If you got it with the PC, then it's likely to be OEM, in which case you'll have to contact the PC's manufacturer