OK, so my understanding was:

You have a desktop Pentium 3 with XP installed on it.
And you now have a Pentium 4 also which you want to
install it on?.

OK firstly you shouldnt be running the same copy of XP on
both machines, that will violate your licence agreement.

1 disc and licence = 1 machine

However, if what your saying is that you have replaced
the Pentium 3 with the Pentium 4 and want to know if the
XP copy (which is no longer installed on the Pentium 3)
will work and what will happen interms of activation.
Then basically it shouldnt be a problem, you can install
XP on the new machine with no problem, just activate it
as you did the first time.  If you have activated over a
certain number of times you may have to call up to
activate, but again this shouldn't be an issue.

Hope this clarified it for you.

Mike Bright MCP, MSP

e:mike.bright@brightweb.co.uk

Greetings --

   Assuming a retail license (OEM licenses are not transferable),
simply remove WinXP from the computer it is currently on, and install
it onto the new one.  If it's been more than 120 days since you last
activated that specific Product Key, you'll most likely be able to
activate via the Internet without problem.  If it's been less, you
might have to make a 5 minute phone call.

   Here are the facts pertaining to activation:

Piracy Basics - Microsoft Product Activation
http://www.microsoft.com/piracy/basics/activation/

Windows Product Activation (WPA)
http://www.aumha.org/a/wpa.htm

Bruce Chambers