Why on Earth would you WANT to run 32bit Xp with a 8 core bulldozer?????
Perhaps XP provides all the functionality needed, and the user is happy in 2GB of RAM, but needs a brutal amount of processing power? There are some applications which don't need much RAM, but benefit from having a large number of simultaneous threads. Putting those threads onto lots of real cores will result in higher performance.
Perhaps the application won't run on Vista / Win7 without invoking "compatibility" hacks?
Perhaps the OP is taking a moral stance against paying any more money to Microsoft?
There are plenty of possible reasons.