permissions issue with shared folders from XP on corporate network

My XP virtual machine is now on our corporate network. When I set up a shared folder and try to access that folder, I get this message:

\\.PSF\MAC Users/James is not accessible. you might not have premission to use this network resource. Contact the administrator of this server to find out if you have access permissions.

I am the administrator of my Mac.

I can set up a shared folder that maps to my Mac's public folder. I can access that just fine. So clearly this is a permissions problem.

My Mac doesn't see the Windows network user "angusj" as having rights to access the folder "James".

I tried setting up a Mac user called angusj but that didn't help. I tried to change the permissions on the Mac folder to allow full access by anyone (as a test), but that didn't help.

Any clues?

I somehow need to tell the Mac that the XP virtual machine has permission to access the folders I share.