Router Help

Hi! My problem is kind of confusing and weird so hopefully somoene can help! Anyway I formatted my desktop computer the other day at a friends house. When we were finshed everything worked fine with my computer connected to the internet fine and all. So when I brought it back to my house I connected my desktop to my router (linksys wrt54gs) it will not connect to the internet. I also have a laptop at my house which is connected wirelessly and my computer that I formatted at my friends is the computer that is connected straight to the router. So when both computers are on neither one of them can connect to the internet. But when i turn off the desktop my laptop can connect to the internet. So I aslo tried to connect my modem straight to my desktop when i did that it worked fine. So i have no idea what the problem is can anyone help? Thanks!