Blue Screen error with user32.dll...help?!

Okay, I am trying to fix my parents computer for them, but am having problems.

The exact error message is this:

STOP: c0000221 {Bad Image Checksum}

The image user32.dll is possibly corrupt. The header checksum does not match the computed checksum.

Using F2 or F10 before it reaches that point I have managed to run tests but they tell me everything is fine. I have read other users threads who have this problem and also the help page for this problem, but I am still stuck.

Firstly, I'm not sure which CD I should be using. I have the installation discs that came with the computer, but I don't know which one has XP on it?

Secondly, the CD/DVD drive on the computer is temperamental (just to be awkward). I have a external CD/DVD drive that I can attach to the computer, but I don't know if this will work as I have not previously used the external disc drive with the computer.

If this won't work is there any way of fixing the file without using the CD drive? :s

Help would be much appreciated - at the moment my parents can still use my laptop to access the internet, but I'm back off to Uni soon!

Thank you. :0)