Cannot acces shared files over Network

We are running a small business network with two vista ultimate machines and
one XP machine. These machine all access the internet seperately through a
router. After changing the workgroup name all computers appear on the
Network. The two vista machines can access the XP machine and all its shared
files and folders. None of the machines can access the vista shared files and
folders, but the printer sharing works. The network discovery, file sharing,
public folder sharing, printer sharing are all on. Password protected sharing
and media sharing are off.

We keep receiving the messgae \\Computer\folder is not accessible. You might
not have permission to use this network resource. Contact the admin of this
server to find out if you have access permissions. Access is Denied.

The share permissions on all the folders are set to Everyone with Full
Control.

Please let me know if there is anyhting I am missing in order to make this
work.