How to back up a Vista laptop over a wireless network dominated by

I hate to bother you, but I'm in the 300 and somethingth position at the
Library for Vista for Dummies... I would like to store my Vista laptop's
backups on one of the network XP'sI've mapped both machines to a MobileBackup
folder (Z on the Vista and the chosen XP. When I go to initiate the backup
on the Vista, I get hit up for a login and password. Which one does it mean?
The initial login? It's asking for info from the XP, but whrn I put in the
login & password, I get an error stating the network share could not be
accessed for the following reasons:

No mapping between account names and security IDs was done (0x80070534)

Please ensure the network location is valid.

Any words of direction?

Thank you!
e.t.