Can connect to network, but not Internet

Hello,

I can connect to the wireless network with laptops and smartphones, but my desktop won't connect to the internet. If I unplug the modem and router and give it a bit, the problem goes away. I stay connected for a couple days, and then it happens again. I ran scans with AVG and Spybot, but neither found any problems. When I run the diagnostic it just keeps suggesting that I reset the IP settings, which does nothing. I can continue to unplug the modem every few days, but I'd really rather figure out what the problem is.
Any help would be greatly appreciated.

Thanks.