This may not help you but it worked for me on some other games. A few nights ago Microsoft did an update for Live and some things to Vista (you said you have a new computer so I'm assuming it came packed with Vista). I could get inside the first lobby menus just fine for Lost Planet on my roomie's PC and my Xbox but I kept gettin' booted about 1-2 min. after I'd join a match lobby (Same for me with Gears of War). Yes, it happened on both my roomates PC (The LP PC player runnin' Vista) and me on my 360. All I did was update my router's firmware and reset everything after doing so. Seems a little odd, I know, but it worked. All of this is assumin' you use a router though, if not, I got nothin' :/