"My laptop, which has been sitting a couple of feet from the desktop at all times, has no problem finding and connecting to my router."
Silly question but have you tried connecting with the PC when the Laptop is switched off in case its [the Laptop's] WiFi is interfering with the PC's ability to connect?
Also try changing the Channel number in the router's wireless settings.
Most routers use either 6 or 11 as their default so try these or, if already using one of them, try the other. Avoid 1 and 2, as these use frequencies shared by other domestic wireless devices. Also avoid 12 and 13 as, in some other countries, these are not legal and consequently many WiFi adapters are designed not to connect on these channels.