hmm, do you mean, slave the hdd in a full xp pc? then i think you'd need an hfs+ reader (if that exists) to "see" the g5 hdd.
if you mean can the xp pc boot from a drive that came out of a g5, i'm pretty sure that's impossible (unless you reformat to nt, which is bypassing the question).
g5 were ppc.
might find related info by googling something like this http://www.google.com/search?q=g5+ppc+|+970+|+970mp++type+%22Boot+ROM%22+|+%22Open+Firmware+ROM%22