MSN Messenger blocked by router?

I've set up a wireless network between an XP machine and a Win 98. Everything works perfectly - apart from messenger! When attempting to sign in it just churns over and does nothing. I've disabled firewalls, deselected proxy servers, enable various ports - just about giving up. The main computer is running XP Pro with AOL 9.0

The router is a 3Com Office connect 11g wireless ADSL

When the router is disconnected and the machine is reconnected to the phone socket via the ADSL modem, Messenger works perfectly. Re-introduce the wireless router and it give up the ghost.

I reckon it must be a router problem - but what?

Any suggestions?

Graeme