Jack,

You said...

I have a Dell xps 420 connected to U-verse wifi. It was working fine

Try using system restore first and see if you can go back to when it worked.

Rick

Thanks. Tried system restore--twice; still no good. Windows Network Diagnostics gives me the following message now: "Wireless security types do not match."

Jack,

What I would do.

How to login to your wireless router go in and change your SSID to a different name. Also use these Recommended Wireless Router Settings, I use channel 11. That way you have a clean start with a better connection. If you do not have any usb device, wireless printers or any games, then you could make the security WPA2-AES instaed of WPA-PSK(TKIP)

Rick 

Thanks. I began to try this, but couldn't find "Default Gateway,"  I decided not to continue looking, because two other computers and my iPod Touch all work fine,and I'm afraid changing the router settings would cause a snafu with them. The settings problem resides with the computer rather than with the router.

Still stumped, but thanks for the suggestion.

Jack