Hi and welcome.
It's not quite clear how the computers are connected. You say that the XP desktop machine is connected via ethernet, but then you refer to signal strength (which implies a wireless connection). I'm going to assume that both PCs are connected wirelessly, but please tell me if I'm wrong.
This sounds as though the wireless configuration hasn't been completed. What you need to do is connect a PC directly via ethernet and then go into the router's web interface to configure it. In the wireless setup you should specify an SSID (any name you like) and the type of encryption to be used. WPA encryption is the best choice here, and give it a good hard-to-guess pass phrase. It's often a good idea also to select a different wireless channel from the default, although any channel should work OK.
Now you need to configure the wireless interfaces on the PCs to have the same configuration as the router, after which you should be able to connect properly.