Is the desktop hard-wired or using WiFi to connect to the router?
Is the fluctuating signal problem on the laptop, or the desktop?
If it's only the desktop, while the laptop's OK, then the problem is the WiFi card in the desktop.
If the desktop is hard-wired, and the laptop will connect OK from 30 feet away, does it still fluctuate in the same way? If so, problem is either the laptop's WiFi, or the WAP - can't tell which without another WiFi device to check with.
A friend of mine came round here with her laptop the other day - she was getting spotty connection if any at all when at home. Same problem here. I plugged in my PCMCIA wireless card, and got immediate solid connection. Off she went to ebay to buy a PCMCIA card (under £20 for the new 802.11g one).