Start regedit.
Go to:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_HTTP_USERNAME _PASSWORD_DISABLE
to re-enable it for the entire machine,
or go to:
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_HTTP_USERNAME _PASSWORD_DISABLE
to re-enable it for the logged in user.
Now create iexplore.exe and explorer.exe DWORD values and set their value data to 0.
Done, you just got the user:pass@ functionality back.
Source:
http://weblogs.asp.net/cumpsd/archiv.../07/69366.aspx
ZoiNk