At work we use the same license key on 7000 PC's.
Volume license or OEM copy then. If each PC has XP sticker on it (the certificate of authenticity or COA) then its probably OEM. I would guess they also say "Dell" or "HP" on the front of them and are installed from a central image by your IT guys and girls already pre-authorised with MS.
As for the original question you can re-install XP over itself. When you boot from the CD and work through it will ask you to press "R" for recovery console. Dont. It will then ask you a bit later on if you want to press "R" for a repair install. Do that. *dont* format the drive, if you get that far you have gone to far! The repair install installs XP over itself. Your apps wont work but your data should still be there.
If this all goes wrong however I take no responsibility.
If you really want to be sure about the data get one of those hard drive to USB dongles off ebay, pull the hard drive and attach it to another PC and get the data that way.
I don't know how easy it is to remove the hard drive from a mac book. I would guess that being apple it might be tricky? No real experience though of apple servicing. A quick google will solve that for you!
David.
(Edit – this might help http://www.macinstruct.com/node/130 . It doesn't look to bad actually!)