kidistight if you're using win XP then you'll also have a XP Firewall. See (advisable not to have 2 active firewalls b/c they can interfere with each other); Firewall configuration instructions (click on link)
Chances are it's a routing issue. If you used port forwarding then you can't use the same port no. for both. If you're relying on UPnP, make sure your router is UPnP capable.