I have it working!
You were right, Jeff, about the router being signifigant.
The problem spread to my other laptop, also a dell, so I imagine it was precipitated by a windows update, though I can't be sure. If this is the case, there seems to have been a delay of a few days (a couple reboots? a few cycles of signing on or off? a cached setting somewhere expiring?) between the windows update and the problem starting. If there is a similar delay after reversing the cause of the problem, that would explain why system restore didn't seem to have helped.
I was able to connect, with WEP, to the router from either laptop when running mepis linux from a livecd, but from neither when running windows, so the software on the laptop was a factor.
But, just to try it, I updated the firmware on the router. Problem solved. Looking through the bugfixes listed with the update, I saw one for compatablity problems with centrino systems. Neither of our laptops are centrino, but there must have been a compatablitly problem exposed by the update to the laptop software, and fixed by the update.
Here are the details:
Dell Inpiron 5160 XP2 Windows Zero Config Dell Wireless 1350 WLAN Mini-PCI Card
Dell Inspiron 5100 XP2 Windows Zero Config Dell TrueMobil 1300 WLAN Mini-PCI Card
NETGEAR MR814v2 Firmware Version 5.3_05 Firmware updated from 5.03
Thanks for all your help, hunwalla