If I read your post correctly, you can still access the internet via one or both of the wirelessly connected PC's at a time when the cabled PC is not responding and seems to be disconnected. If this is the case then as you say, it cannot be the modem. The first thing to check is that your network card is properly seated. Next check the cable to the PC that is "disconnected" by trying another cable. After that, unless it is a dodgy ethernet port on the modem/router (try the cable in another port if you have one) it seems it has to be a windows problem - perhaps one of the services windows runs in the background is interferring on occasion.