Cannot logon network share resources at some clients

Dear all,

I have four windows xp pro sp1 pcs. All of them have network shares
that enable others to change.
I have access problem to PC1 and PC2 with the following error:

\\PC1 is not accessible. You might not have permission to use this
network resources. Contact the administrator of this server to find out if
you have access permissions.
Logon failure: the user has not been granted the requested logon
type at this computer.

Information:
1. All of the PCs use the same workgroup name
2. All of the PCs use the same network (192.168.1.X) and subnet
3. All software firewall have been disabled in all pcs
4. All PCs could be shown in Network Neighbour
5. PC1 and PC2 can access freely to PC3 and PC4
6. Error occur when accessing (By UI / UNC) PC3 and PC4, except access their
own, eg. successful when PC1 connecting the \\PC1
7. All of the PCs have the same users (administrator privilege)

Regards,
Taboo