My guess here is that you actually have two firewalls.
One in the router and one in the Speedtouch modem itself. I know that the Speedtouch 510i comes with built in firewall/router so I wouldn't be surprised if your modem is the culprit.
How to check out;
connect you computer directly to the modem. and start. Or you can choose start, then choose run.
In the run window, type "cmd" and hit enter.
In theblack window appearing type "ipconfig /release" and hit enter.
Then type "ipconfig /renew"
if the IP you get is in the 10.x.x.x / 192.168.x.x or 172.16->31.x.x your modem has an DHCP server too. Easiest is then just to get rid of the router and get a switch. Or you will have to forward every port in use first to router, then portforward from there.
Another option is to set the modem in bridged mode, basically it means that its dhcp server and firewall is disabled, and all data coming in is just sent on to the router.
http://www.speedtouch.com/prodprofwshdsl.htm
If its this one, it has its own build in firewall & DHCP server, so you actually have two firewalls to configure.
Hope this helps.
GenRabbit