When I rebuilt my main desktop recently, I mounted the drive from my old desktop too, so I could read the data from that, but to my surprise I found that from my new Windows XP installation, I could not read the files in the 'Documents and Settings' folders on the old drive.

After a lot of trawling round the net, I discovered the answer:

How to take
ownership of a file or folder in Windows XP

I'm going to take a good backup now, I promise!