BleepingComputer.com: 1 Of 3 Pcs Cant Detect Wireless Signal

I've got a wireless home network (just internet, no file sharing) with 3 PCs. During the last week or so, the oldest PC hasnt been able to detect any signals from the router. The 2 others are, as far as I can see, working as normal.
Does anyone have any ideas why all I get is "no signals detected"? If the PC finds a signal, and it rarely does, its gone after a few seconds. The computer's old, slow and a bit unstable, but this hasnt been an obstacle in the past, so I assume this isnt the reason? The computer's OS is ME and both the router and the wireless card are made by Belkin.