OK, I finally read the first post (kinda helps) and have a little more of a clue as to what you would want to accomplish. First, I see nowhere in this process where you would need to use \\ and \ as these are generally use for UNC. As the fields are already labeled, they wouldn't be necessary.
It SEEMS that the problem is one that you would run into as a peer-to-peer. You don't necessarily have permission to access the XP computer even if the main computer is an admin. This is one of the downfalls of P2P. With single signon on a domain (when you log into your computer -- using a username and a password), you are now logged into everything that your admin or you lets you.
This brings up why it may work when you change the setup on XP. Once, you can access it, you may be able to run the backup. One key thing. On the folder that you designated to be the backup folder on XP, is it shared with full rights for everyone for the time being: Give "everyone" share rights and "everyone" full permissions? You could tighten up the permissions later. Actually, Microsoft recommends giving everyone share rights, which is weird, but after, you should only use domain users or authenticated users and NEVER everyone on permissions.
One thing that would be good to try is to use an external hard drive rather than back it up to another computer on the network. Network backups in general are shaky and even shakier on another computer.
