ADSL Gateway usage with Win 2K

Our network currently uses an ISDN router to access the internet who’s address is allocated by a Windows NT4 Server. We now have a firewall and a ASDL connection as well, having just changed a Windows 2000 pc to a static address with the firewalls gateway address I am mytified as to why this pc still uses the old router for internet access. IPconfig states that my settings are correct. Anybody got any idea’s?