The four things you looked at don't seem like they would have anything to do with it especially the ports, etc. They generally let something in or don't. Having said that, if you have McAfee or Norton suites on either XP Pro machine, I would UNINSTALL them.
Any company such as Acronis or Backup Assist will tell you to upgrade before almost anything else. I would upgrade to v6.2x.
I doubt it is anything on the server, although hard to tell.
The other fix would be to buy two more WIN 7 Pros and get rid of the XP Pro. This is the perfect situation where having mix and match makes troubleshooting more difficult.
