It may be that the unit has just failed. Although uncommon, it is not unheard of.
You are welcome to contact D-Link's technical services, see if you can get a replacement,
or you and I can try to work it our together here if you like, the option is yours.
I look at the security levels of the browser, see if you can allow the IP of the router to be a trusted site. Also try lowering Vista's IE security to low, just to see what happens.