What were the specs of this XP box? Because Windows 7 isn't magic. It's not going to suddenly revive old hardware.
Did you check the minimal requirements? You need at least 1 GB of RAM, for example, to have a smooth installation. And even then, you'd need a processor likely no older than a Pentium 4 and some sort of integrated or dedicated GPU to run Aero Glass.
By all accounts, Windows 7 runs faster than both Windows XP and Windows Vista on the same hardware, but all of these tests were conducted on machines that were "Windows Vista Capable." So, again, what are the specs of this computer, and how old is it?
Also, Mac OS X would probably run horribly on that box, as well.
3.2 ghz Pentium. 4 w/ 2 gb ram
vista practically killed it
and leopard runs fast on all the macs i have. including a 1.33ghz g4 powerbook