Windows 7: Can't play LAN games when internet is enabled

I'll try keep this as simple as possible. Basically we have a two-machine LAN (all fully working, both show up in the network browser, no internet connection sharing is enabled, file sharing fully working, pingable etc with static IP addresses), and when we start one machine running a LAN game (e.g. Battlefield 1942), the other machine can see the game and connect to it with no problems... but the minute we enable the internet connection (crappy 3G mobile broadband), the LAN games no longer show up. It doesn't appear to be the firewall, as even with it entirely disabled the exact same thing happens. Internet enabled, no LAN games can detect each other. Disable the internet, LAN games show up.

I had heard of this showing up in earlier builds of Windows 7, however both of these are the final Ultimate and fully activated so it's kind of a serious problem.
Any suggestions?

Hi

Is it safe to assume you havw 2 pc's connected thru a router?

Ken

No router. The two PC's are directly connected via ethernet.

No router, pc's connected with ethernet, how do they connect to the internet?

Either one can connect via a 3G mobile broadband dongle.

are you using a null/crossover cable for the two pc's or is it stright ethernet cat 5?

Ken

Cat5. We have been using this setup for a long time for LAN games (Server 2008 to XP and before that, XP to XP, before that 98/2000). The only new element is Windows 7 and it is very specifically related in some way to going online.