|The external drive you have should work on both vista & xp.|
Plug it back in to your vista PC & see if you get the same message?
If you do, it's a problem with the drive, not with xp.
If you don't get that message on vista, then maybe your XP is not using the latest service pack (SP3)?
SP3 can be downloaded here for offline installation: http://www.microsoft.com/download/e...
Or to avoid such a large file download, use Windows Update which will only download the bits of SP3 that are relevant to your PC.