Need help with odd disconnect (and can't reconnect) problem

My roommate keeps getting kicked off of the wireless network despite having a strong signal which isn't the major problem but if you can help with that great. The MAIN problem is that every couple days or so he won't be able to reconnect at all. He can connnect to an open wireless signal in the area but not mine anymore. The really strange part is that he can fix the problem by doing a system restore from 2-3 days earlier. This works but after a few days the problem happends again and he has to do the restore again. I asked him if he's installed anything new since he's done these restores that might explain the change but he said he hasn't made any changes to his system.

He's using XP and we're running Sympatico DSL through one of their Speedstream modem/routers. Also I should point out that I have 2 other roommates using wireless and this is NOT happening to them.

Thanks!