Yes, if your bios lets you boot to USB devices, and there's an install of XP on a harddrive that you can plug in via USB, then yes, in theory you should be able to boot it.
The problem though, is that windows is set up to use your old hardware... It's assuming you have X brand of chipset, and X brand of IDE controller, and sometimes (quite often) you'll end up booting off the external drive only to find yourself with a blue screen of death saying STOP 0x0000007b.
I've found a workaround to patch the registry to accept the new hardware, but I've had mixed luck with it. You can have a look
here, though again, I've only had that work a couple times. You could also try deleting the HKLM\System\CurrentControlSet\enum key from the registry, I've had that work a few times as well, though this sort of thing is well beyond the experience of most users, and like I said, it really only has a 50% chance of working.
