Depends.
If the copy of XP that came with your old computer is an OEM version, it's
supposed to be non-transferable. (It may even be BIOS locked to install only
on the same make of PC.) If you have an OEM copy, you'll need to obtain a
new license. (You could get another OEM license if you bought it with the
new PC, but you'd have a similar problem at your next hardware replacement.
I'm unsure how it works with a generic OEM license; it should be possible to
do major upgrades, like CPU and hard drives, that would trigger activation,
but still should lie within the definition of keeping it on the original
system.)
If it's a retail version (upgrade or full), you can do it, no trouble. You
are supposed to remove it from the old machine, but there's no
de-activation procedure. If it has been more than 120 days since the
(retail) copy has been activated, you can activate it on the new machine,
like it was a new activation. If it has been less than 120 days, you'll
probably have to do a telephone activation, and explain to an MS rep what
you're doing. It's a 5 minute call.
HTH.
Bob Knowlden
Spam dodger may be in use. Replace nkbob with bobkn.