Setting the Host and Domain in XP

I’ve recently installed a new PC with XP on the network. (Our first one!) The PC needs to talk to two networks, I’ve tried using the secondry IP address in the Advanced IP settings but that didn’t work.
I then installed a second NIC and have two IPranges. I can see both networks but get various errors when trying to log on to some network programs, this is resolved if I disable one of the NICs.
I think the problem lies in one of two areas (or both:
1. When I try to join an NT Domain using known good ID and password, I get a message saying the user id is invalid.
2. I can’t see a way of setting the Host name and Domain under DNS, I’m very much used to the way it’s done in WIN98.

Big points on offer because I need a quick answer.
Thanks.