Sharing this experience in case it's of use to others.
Trying to configure a desktop PC with built-in Intel wireless capability. It 'saw' the wireless LAN but couldn't connect. Three laptops in the same room all worked fine. Occasionally (just twice actually) the desktop PC would connect OK after a long delay, but if you rebooted it wouldn't reconnect.
Tried using both Intel's config utility and the built-in XP tools - same results. Tried updating the Intel drivers - no change.
Running short of ideas, getting close to sending the PC back under warranty, I tried changing the transmission channel on the wireless router. Bingo! The desktop PC connected, repeatably and reliably, and the laptops continued to work just fine.