This should work for you You are prompted to enter your credentials when you access an FQDN site from a computer that is running Windows Vista or Windows 7 and has no proxy configured
To accomplish you should put following registry keys to your system:
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers]
„C:\Program Files\Microsoft Office\Office12\WINWORD.EXE“=“WINXPSP2“
„C:\Program Files\Microsoft Office\Office12\EXCEL.EXE“=“WINXPSP2“
„C:\Program Files\Microsoft Office\Office12\POWERPNT.EXE“=“WINXPSP2“
Be care with the slashes. If you use a reg file you need to use double slashed \\ instead of one slash \.
If you have no access to the HKLM hive you can use the HKCU hive instead.
This is a known issued. If above solution do not work and for more details about this issued check following blog entry at msdn Office 2007 prompt for user credentials.
Timo