For a situation like that, I think VirtualBox will be your best choice. See my
http://home.nyc.rr.com/computertaijutsu/vmcomp.html
(Did I mention that page already in this thread? Well, it was probably on page one.)
The only argument against it would be if you need the XP machine to be a host on the LAN. However, setting up a bridged connection isn't overly difficult.
VMware-server (choose the current 1.6-whatever, not the beta 2.0) does the bridged networking out of the box but requires more resources in my opinion.
Lastly, one thing I do find is that any VM will gradually eat up resources, and I've found that if I start with nice -n 19 (lowest priority) it will still respond well enough, and not slow down the host very much.