Can't get LAN games to show up

[STKD]

Fairly basic. We have a home network setup with the following:

"Home Fileserver Machine" = 192.168.0.2

PC1 = 192.168.0.4

PC2 = 192.168.0.7

 

(fileserver runs 2008 standard, both PC1 and 2 have XP and 7 installed)

 

Both of these machines connect to the fileserver machine (this has no firewall or internet connection at all - just local), all can ping each other etc as usual, and both PC1 and PC2 have their own internet connections. We can play over Live but are stuck with terrible "mobile broadband" and need to get LAN play working.

However, if one player creates a LAN match, it never shows up on the other players browser no matter what settings we try. GTAIV and all the relevant parts have full access past the firewall. Are there any extra ports that need shared or anything like that? There is very little out there about LAN mode at all so I could really use some advice on this one...