Just got back from recovering files from John's hd.
He said that he'd just been doing normal stuff like opening some documents. Then his hd made some clicky noises, the computer froze, and then died.
Normally, when one starts a computer up, it will make a click noise, then the power-up noise.
When he tried to start his computer up again, it just kept making that click noise until he put the WinXP install CD in. Apparently he tried heaps of stuff.
When my brother and I got there, we plugged his 40GB Western Digital hd as slave into our own computer. It was detected, but was not formatted. We tried a data recovery thing, but it was going to take too long so we stopped that. Put in the Knoppix Live CD, restarted, and lo, there was the hd, with all the stuff on it! We tried to transfer it over to our own hd, but it said that since our hd was formatted to NTFS, it wouldn't do that in case it wiped everything else off. Then we tried transferring the stuff to the USB flash drive, that didn't work until we set the permissions properly. After that, it was drag and drop, then unmount and shutdown.
John turned on the other computer to check that the stuff was still on the USB dongle and whether it was accessible. It was.

The entire thing took an hour (because we wasted about 10 or 20min waiting for the recovery process to find something), and we were very pleased with ourselves. I think John's family (they have 3 ppl) were just sort of confused.
I suspect that his hd had its partition table corrupted, since the data was all there, but WinXP said it was a blank hd. I also think that Knoppix determines the partitions by actually reading a section of data on the hd, rather than just relying on the partition tables. I'm actually not too sure. Should ask about that. But if it does actually rely on partition tables, why couldn't Windows read it? Is there some Windows-specific thing that determines whether partitions are there or not?

Yes. Very pleased indeed. Even though it was actually Knoppix that did all the work.