Is the XP disk you are using OEM. If it is, you are out of luck. If I'm reading this correctly, you said the laptop had xp, but was it from the disk you are now using or from their work copy?
If the copy you have now is OEM, it is only good for the motherboard on the computer that it came with and will not work on others. So, for example, say you got the disk for a dell and it was an OEM disk and no a retail version, it will only work on that dell. If you change the Motherboard in the dell, it will not even work on the dell. Microsoft will not help you and will refer you back to who you got the computer from. On OEM versions the license is on the machine (and only actually the original MOBO) and not an individual.
If this is a retail copy, or one that came with the actual laptop, then I would use the phone activation mentioned earlier.
Another problem with work computers, if you got the xp disk from his work, is that the copy of xp they gave may be OEM and from another computer. Alot of IT departments, although license every copy through either OEM or regular licensing channels will buy a seperate retail version to use on all computers, so as not to have to keep track of all of the disks. When unloading them, they just grab a disk to throw in with the laptop. If it is OEM and not for that exact laptop, it will not work.