Default Domain in "Windows Security" login dialog

When using IE8 and navigating to a site that requires authentication, a dialog is poped up requesting the username and password. On Windows versions prior to 7, this would not default the Domain to the name of the computer (assuming the computer is not part of an AD Domain), in fact there was no default domain name used as part of the username.

On Windows 7, the Domain defaults to the user's computer, if the username does not contain a domain specification ex not like: "MYDOMAIN\myusername". If you just type in "myusername", then the default domain is explicitly passed. If the site assumes that just the username will be passed, and infers the domain, the way Windows 7 always passes an explicit domain name (either typed as part of the username, or the default computer name) causes the login to not work. In my particular case, we're no quite sure what domain name the web site is expecting (since it expects just a username and infers the domain for us).

In Windows 7, when the "Windows Security" login dialog pops up, how do you tell it to just pass the username, excluding the default domain name?

I just started using Windows 7 and I am experiencing the same issue with the default domain in "Windows Security" login dialog? I've been searching, but no solution so far.

Some guidance would be greatly appreciated.

Thanks.
Bud