the xbox can get infected depending on the viruses, I would recommend not connecting until this is all cleaned up. You could always format the computer, if its crashing and running so slowly and you can't do anything, then that could be a major problem. It would be hard to fix because everythign would be running so slowly. Remember, doing this would loose all your data.... ***all of it***. You could also try a system restore, returning your computer back to a previous state when you had no viruses. The problem with that is that you loose everything you have done from that point on.
Many trojans and viruses cannot be removed by anti-virus programs. try searching the name of your virus then "fix" after it in google, download any programs that look helpful, and run them. I recently had a couple trojans/viruses that took ages to get rid of, even with my anti virus (AVG).
A Hijackthis log is very important to get, they can make or break your chances of getting rid of these threats. If the computer freeses while trying to do this, reboot and try again. The people on this site know how to work these and can help you out a lot more if you can get just one log onto here.