Yes - the risk is you would be pirating the software.
If you require Windows XP on the second computer then you must buy another
license/copy for that second machine.

Don't think of the Windows opertaing system as something you just put in a
PC and use then put in any other (like a DVD movie of music CD).
Think of it more like a part of your PC (like your CPU or memory).
Yes you can take it out and put it in another PC but if you want 2 working
machines you buy 2 CPUs and 2 sets of memory.
And thus 2 copies of Windows XP.
(This is the same license as we have had for all versions of Windows since
Windows 3.1)