The one nagging doubt I had regarding the process relates to a registry setting on the XP box to bring the security level on the file sharing down a notch. I had a quick shufty round the web but couldn't find it last night, believing it to relate to some LanManServer settings (but this was all from memory). However, this linked thread should bear a striking resemblance to the question you've been asking (and you can see the answers are roughly on a par). It also starts talking about registry entries in the first post. I don't have an XP box to hand to check the current existence or setting of the registry key, but maybe someone else on here can check for you, if you're unwillling to go that far into technical things:
GUIDE: Networking Windows with OS X - MacRumors Forums