The answer is no, even if I havent tested it. you can do that under OSX , but not on xp.
Nevertheless, boot camp drivers modify the "startup" menu in xp (it looks exactly like the OSX one), so that you can use target disk mode, but I'm not sure it's for xp, maybe just so you can restart in OSX using firewire disks. So I may be wrong in fact, but i'm 90% confident in what I say.