Yes, Matthew, steer well clear of friend's PCs, and, even more so, relatives!
I once tried the relatively simple task of installing some additional memory, and a new graphics card in my cousins PC. I had just built my own PC without any trouble, so didn't expect any problems, but something went very wrong, and we ended up having to use the restore CD which came with the PC.
Unfortunately it was a restore CD for ME, not XP, which we needed, but by the time we realised, it was too late, and the whole system was knackered! She then had to 'borrow' an XP CD, and do a complete reinstall, losing all of her unsaved data, but again, something went wrong, and her main drive is now 'E', not 'C'.
Needless to say I have not touched her PC since!