Can't backup to drive on XP from Vista

Don't know why it should be so hard to use my machine. I have tried and
tried to backup my Vista Machine to my Windows XP Pro system. After 6 weeks
of on and off attempts, I have had to resort to backing up to my Vista
Machine and then copying the backup folder to the XP Pro system. If I can
access the XP Pro system to copy, why is it required to have a user name and
password. There is no username and password on the XP machine and never has
been. The Drive is set to Shared, the Folder that I wish to back up to is
set to Shared.

I constantly preach Backup, Backup, Backup to computer club members. Now, I
am the one not doing it.

Is there a solution?