You might need to borrow a friend's Xbox, but here's a link that should help out.
I'm trying to link my X-Box Live account to my Windows Live account so that I can play Fallout 3 on my PC with my old Gamertag. I don't have an X-Box anymore and I'm trying to figure out how to do it on my computer if it's possible at all. I can't find anything about it anywhere. Any help I could get would be great.