I can access only one shared folder on a computer

[ILS]

I have four computers (call them C1, C2, C3 and C4) hooked up together via a Westel router. C1 and C2 are Win XP pro, C3 is Win 98 and C4 is Win XP home. All have the same workgroup. I’ve right-click ‘shared folder on the network’ for a dozen or so folders on each computer. Using network neighborhood, I can see all the folders that I’ve shared. From C1, I can read all of the files and folders I’ve shared on C2, C3 and C4 . However, from C2, C3 and C4, I can get access to only one of the many shared folders on C1, ‘my documents’. (note that I have two administrator users on C1 and ‘my documents’ of both are shared, but only one of them actually shares.) When I click all other nominally shared folder, I get a message like: “<such and such> document is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions. Make sure the disk is not full or write-protected and that the file is not currently in use. Access is denied.”
Further details:
All names are correct and all permissions are given. I’ve tried with and without Simple File Sharing" Enabled. Firewalls should be no problem since one of the folders on C1 is sharable.
I am using zone alarm. I’ve disabled it, and that didn’t make a difference.
I’ve runNTrights.exe is in the free Win2k3 resource kit, and that didn’t make a difference..
Can anyone suggest what’s wrong with computer C1?