I'm trying to run the XNA Client/Server example over a LAN network on some computers at my school.  I'm running into a crash that seems to happen randomly (sometimes it takes a minute to crash, sometimes it takes 5 minutes):

 Unhandled exception at 0x02a20b20 in XnaLiveProxy.exe: 0xC0000005: Access violation writing location 0x0000f319.

I've run the same code at home over two PCs and had no problems (I've had a solid connection up for over an hour with no crashes).

I don't have administrative privilieges at the school so I'm not sure if that's what's causing the crash or if there's just some sort of port that needs to be forwarded for Live.  If anyone has run into this problem before or has any idea what a potential solution might be, I'd really appreciate your help.

The systems at school were set up with XNA Game Studio by an admin correct?  At what point do you get the error?  At startup, when bringing up the guide, when signing in, when creating a session, etc ?

Problem solved.  It turns out that the computers at the school just needed the newest version of Games for Windows Live installed (which is redist v1.2 at the moment).  So if anyone else gets a crash like this (which happens randomly once you're actually in the network session), that's probably what's causing it.