Cant host ANY Games?!?!

Im going to a LAN party in a month or two so im trying some games we can play,and no matter what game i try,im getting some strange hosting issues!

My network...

PC2------------- |---------->Router<-----------Internet

PC1:Xp Sp2:Wifi
PC2:Xp Sp2:Ethernet
PC3:Vista Home Premium:Ethernet

Now none of them will ping to each other,and i can only host games from PC3.If i try to host from the other computer,none of the computers can see the game or if they can,they cant connect due to timout issues (or Server not responding).

Also,if i view the network in explorer all 3 computers come up ONLY if all 3 computers are on and connected.If i disconnect PC3,none of the computers will show up at all until i reconnect PC3 and restart them all....

Anyhelp would be GREATLY Appreciated :(