First, I have to note that upon entering the first Vista machine into the XP flooded home, I was all over this section asking for help. It took a bit of legwork on each machine to give with one, so running the Win7 Betas just seemed like an easier way around all this; and it was. IMO, it's about that homegroup password that isn't in XP (Pro, right?). The best I can say, and that ain't much, is to go into the Vista box's Advanced sharing and disable most of the security features. Turn on Network discovery, file and printer sharing, sharing so that anyone with network acces can read and write, and just 40-to-56 bit encryption since there's nothing lower (that I remember). At that point, you still have to deal with a homegroup password. I forgot which way it was, but I believe XP could go into the XP box but the Vista box was dumb.
But I can help you with the log-on screen. First, give yourself a password in user accounts. Make it simple or strong, doesn't matter if you have no password now (I suggest one. One of mine is qwErtY409 and isn't hard to forget for me). Now, since this is XP, run "control userpasswords2". That'll open a GUI where you'll uncheck "users must enter a user name and password...", Apply, then it'll pop open another box for you to fill out what all users should log in with automatically. Enter your account and password you must made up, apply/OK, and you're done. Now when you reboot, no matter who's at the computer, it goes into that account with no user input needed.
Sadly, I don't have a good answer on having Vista or Vista2 network well with XP boxes 100%. I'm a networking mongoloid.