Shared Drive Problem

Main computer running Vista Home Premium with wireless DSL router.
Secondary computer running XP-SP3? (all updates done). This was my old
computer that I wiped clean and did a new clean install and did all updates.

Everything works great and able to print from XP computer with printer
connected to Vista computer. Internet works.

Problem is the external USB drive connected to Vista. I have the drive set
to share. I can not get the XP computer to see the USB drive which I would
like to use as my backup drive. I am using Windows Live One Care on both
computers for the Firewall, virus, and backup. On the XP computer, it says
that I do not have the rights to change the backup. I don't understand this
as I am the only installer and did not set up any user accounts on either
machine. On my main Vista computer, WL shows the XP computer as up to date
but says the backup has not been done. I can not set it from the Vista side
and the XP side won't let me change it. Any help with this?