I just finished installing XPMC onto an external USB drive for my main (Linux) notebook computer. I don't actually use XPMC for much (PhotoShop and DVDFabPlatinum), which is why I put it completely on an external hard drive, rather than waste space for it on the internal 160GB drive.

Apparently XPMC is XP-Pro + MediaCenter apps, but I wouldn't know much about that.
Bloody pain to install it, though -- took four hours to get it on, fixed, updated, and drivers located, downloaded, installed, garbage uninstalled, etc.. Still no apps on there yet. Blah.

Cheers