Vista - Need info backing up a Vista laptop onto an XP based network.

As luck would have it, I have the rogue Vista, which is proving itself mre
than unhappy about sharing space on our home network wirh a bunch of XP's.
My first backup basically took up the whole D: drive, so I'd like to back up
my Vista machine to one of the XP desktops (The one we call the "Base Unit"
because the router is run off of it and also has the most free space, making
it my best alternative. I created the MobileBackup folder in the Base unit's
SharedDocs folder. As I proceeded therough Vista's wizard to point to this
Backup location, I started getting prompted for some signin and password. I
went through the combos for both the Vista and the XP but the wizard wouldn't
take either. Does anyone have the solution or a link that will provide the
solution? As with Adele, security is an issue with a house full of kids that
I'd rather not let play in my stuff. I thank you in advance,
ellen_t123