Are you saying that you
do have XML 4.0 Service Pack 2 already? Most computer users don't know whether they have XML at all, let alone which version and/or update - so good for you.
If you don't have it, have you checked out this topic:
Problems when downloading prerequisites?
If you do have it, XML 4.0 Service Pack 2 consists of a couple of files and maybe you don't have one of them or perhaps one of them has got corrupted. It would probably be worthwhile reinstalling XML4 SP2, in which case see the same topic.
- Wayne