you can use a XP disk to reinstall the OS multiple times all you need is a different XP key code for each time you install it on a different computer
so with their computer all you would need is the disk you have and either a new XP key code or their code off the case, it should be on the sticker that says XP,
but if they dont have XP on it youll just better go to a store and get a new copy of XP
eh and if it was a custom built, the xp cd should have the key unless you lost it. if it was a a custom built pc, find all the driver cds and start installing, if it was like a dell,hp,compaq,etc find the driver disk and install
and if you dont have the driver and if it was from a company they should have their drivers for the computer over the internet for download, buts thats if you dont have the CD
If you only have 1 CD key and its the CD that was used to install XP onto that computer, then all you have to do is call up MS and say "I formatted my computer and the Activation Key is only valid once." or something.
The drivers must be from the hardware manufacturers website.