The biggest problem you'll encounter trying to install XP onto a recent Vista machine is that if it uses a SATA hard drive then the XP installer won't recognise it. Fact is, XP is not SATA compatible and so you'll need to track down XP SATA drivers appropriate to your mate's machine. If it's a laptop, it probably won't have a floppy disc drive either which means that even if you do have the drivers, you'll have no way of feeding them to XP during the initial install process.
I had this problem when I switched my Vista laptop to XP recently. I did, however, find a solution. What I ended up doing was to use a use a program called nLite to shoehorn the SATA drivers onto an XP install CD (copied to HD) which I then cut to CDR. After doing this, it worked a treat!
nLite can be downloaded here...
nLite - Deployment Tool for the bootable Unattended Windows installation