Windows Vista Backup Vista needs password

This may have been discussed I didn't take a lot of seach time looking but I'm hoping someone has a link or answer:

While backing up one vista machine to another, a network T drive, I get this prompt to enter my user name and password. I have no UN or PW, my question is I have set all permissions and access and can save data to the drive and create folders, why does the back up utility request a username and password from one machine to another when none is used and both are admin accounts? I'm stumped and nobody seems to have answers...any help would be supportive.