Depends on if it's the cd is for a branded machine and it came pre-installed on it.. i.e. If it's a HP machine, the reg code sticker on the PC base will say HP and then the reg code underneath, it will then only work on that model of HP as the copy of windows is set up for that pc.
If the Windows XP Software was OEM without a pc, then it's a universal install on any and, in theory you should be able to use the same CD on both pc's. However, I think sharing the same key on 2 machines is, in Microsofts eyes piracy.. Regardless of if you own it.
*edit - what MrAtego said
lol*