Virtual PC 2004 SP1 is available.
Mike Gunderloy beat me to this [0], but SP1 is now available for download [1] now.
You'll really want to get this version if you're trying to run Windows XP SP2 as a guest.
Other reasons are the pre-compactor to help you shrink dynamic disks and a plethora of bug fixes.
However, contrary to some public speculation, the service pack DOES NOT include USB support for guests.
End of line.
[0] http://www.larkware.com/Articles/TheDailyGrind480.html
[1] http://www.microsoft.com/downloads/details.aspx?familyid=b07c9ef0-265a-4237-ae3b-25bc8937d40f&displaylang=en