Hmmm... It's been a while since your post. I'll answer anyway.. :P
It could be a few things. If you have insufficient resources to run the game in multiplayer.. that'll do it. I'll assume single player is working fine for ya. Maybe a firewall up... if you've installed a MOD map that changed your original SWBF2 installation files.. that may make your game unstable... You'd have to reinstall SWBF2 for that one. Does it work at all? ...or only some servers you join?