Can share files with one WinXP PC but not another

PC1 - Vista Ultimate
PC2 - WinXP Pro
PC3 - WinXP OEM (embedded in a machine controller)

The Problem:
PC1 and PC3 can 'see' other PC when browsing Network Neighborhood but
always fail when trying to browse files/folders on the other PC ('no
permissions' or some such msg).

What works:
PC2 can see/share files/folders with PC1 and PC3; no problem

All have given 'everyone' full control on file sharing permissions and
security
All can browse Internet

All have same Workgroup name
All have 'consistant' IP/Sub-net addresses (192.168.22.x)
All can ping each other

All have a 'common' account name ("Tony") with same password on each
machine.
All have run latest AntiVirus (TrendMicro) and Spyware (Spybot)

Any ideas?

Thanks.