Joy finally got around to setting up her shiny newish Windows 7 laptop to use the WiFi access point.
Select the network by name. Select "Connect automatically." Paste in the WPA2 key (a great long hex number).
"The key or passphrase is incorrect."
Eh? It's the same flippin' key I used a couple of days ago, with one of my Linux laptops.
Much searching. Google turns up a few others with the same problem... accompanied by answers that are patently wrong. (No, reconfiguring the router is not the answer. I like the way the router is configured, and it's clearly not a router-side problem in any case.)
The actual solution? Connect manually, and select the appropriate settings (in my case, WPA2 and TKIP). Apparently Windows 7 defaults to something else, and just barfs uninformatively (indeed, misinformatively) if its default turns out to be wrong.