Just a shot in the dark here, but could the router be pooched, ie the eprom that stores your settings maybe is not "e-ing" and "p-ing", just "rom-ing." ?
Here's a thought. Reset to factory defaults with the little pinhole button on the bottom of the unit. Then, log in to the unit. Do not change the username and password. Do change something else, anything else. Monkey with some settings. Make sure to apply them and save them. Then power it down and back up. Log in. See if the settings you changed stayed changed, or if they defaulted. If none of the settings change, the EPROM is pooched, and you need a new unit.
If the settings did change, try the username and password change again. If that doesn't change, then I suspect that your firmware upgrade glitched somehow. Download the firmware again, fresh from their site. Save as new file on your system, and redo the firmware upgrade. I had trouble upgrading firmware on that some router model a year and a half ago, and I was using IE to do it. When I used Netscape, it went fine. Maybe try a different (non-MS) browser to push the firmware to the router.
Just some random straws to grasp at, maybe one of them will help.
Dexter...