Vista machine can't connect when XP machine is already running

I've searched around many sites and am at wit's end.

I have a Linksys WRT54gs-Speedbooster router. My machine is a Vista gaming system, the roommates (dinosaur) is XP sp2. I also have a mobile device that can connect to the wireless. Both machines are hard wired. Up until the last 2-3 weeks, my roommmate, who plays SL all the time, I could turn on my Vista machine and log onto the internet. Now if his machine is on before I get home, I can not access the internet with my Vista or Mobile Device.

Now it gets even more tricky. If I unplug his computer from the router, reboot my Vista and mobile device both connect to the internet, and can plug roommates XP back in and he is connected to the internet without a reboot. I've checked to see if his Firewall is off. It is. He's running PC Tools AV.

I attempted to turn on my Firewall to see what would happen and the same thing happened to him. He was unable to connect to the internet until I turned my Vista firewall off.

It seems to me that there is a change on his computer that is not allowing my Vista and Mobile device to access the router, even though when I log on and access the LELA it shows green from my computer to the router, and router to the internet(cable modem) is green.

Update: Now in the last 5 days my router is showing my vista machine Red Lined even though we've unplugged the XP machine and I can connect (Vista) no problem.

Think the router is on its last leg or possible a software issue? I've attempted to turn off IPv6 (but it was on before with no problems). This is a new issue. I believe the Linksys WRT54gs is v6 (or v5).

Thoughts?