I just read the windows xp eula (I didn't have anything else to do at the time) and I read this:
* Installation and Use. Except as otherwise expressly
provided in this EULA, you may install, use, access,
display and run only one (1) copy of the SOFTWARE on
the COMPUTER. The SOFTWARE may not
be used by more than one (1) processor at any one time
on the COMPUTER, unless a higher number is indicated
on the Certificate of Authenticity. You may permit a
maximum of five (5) ("Connection Maximum") computers
or other electronic devices (each a "Device") to connect
to the COMPUTER to utilize the services of the SOFTWARE
solely for File and Print services, Internet Information
services, and remote access (including connection sharing
and telephony services). The five (5) Connection Maximum
includes any indirect connections made through
"multiplexing" or other software or hardware which pools
or aggregates connections. Except as otherwise permitted
below, you may not use the Device to use, access, display
or run the SOFTWARE, the SOFTWARE's
User Interface or other executable software residing
on the COMPUTER.
So it seems if you install XP on a dual processor machine you are breaking EULA. I got this from C:\WINDOWS\system32\eula.txt
* Installation and Use. Except as otherwise expressly
provided in this EULA, you may install, use, access,
display and run only one (1) copy of the SOFTWARE on
the COMPUTER. The SOFTWARE may not
be used by more than one (1) processor at any one time
on the COMPUTER, unless a higher number is indicated
on the Certificate of Authenticity. You may permit a
maximum of five (5) ("Connection Maximum") computers
or other electronic devices (each a "Device") to connect
to the COMPUTER to utilize the services of the SOFTWARE
solely for File and Print services, Internet Information
services, and remote access (including connection sharing
and telephony services). The five (5) Connection Maximum
includes any indirect connections made through
"multiplexing" or other software or hardware which pools
or aggregates connections. Except as otherwise permitted
below, you may not use the Device to use, access, display
or run the SOFTWARE, the SOFTWARE's
User Interface or other executable software residing
on the COMPUTER.
So it seems if you install XP on a dual processor machine you are breaking EULA. I got this from C:\WINDOWS\system32\eula.txt