Is XP firewall the only firewall you have? If yes then configure it as described
Firewall configuration instructions (click on link)
But it does sound more like a modem/router issue. Can you use the browser to open up the 3COM web page? I think the default web address for 3COM is
http://192.168.2.1
If yes, check the options to see whether there's a UPnP (Universal Plug & Play) option. By default, I believe the UPnP option is OFF for the 3com devices. You'll need to search around the options for it. I have provided a sample image from a different model 3com device. I believe if you have this option on yours then it will be in the Firewall>Advanced settings.
If no, then manual port forwarding is probably the answer.