Access Denied (Time based, SMB, PDC)

Well, let me say this right up front, this is a non-trivial complicated setup. In this setup I have 2 Desktops and 1 laptop. Both desktops running Windows XP and the laptop is running Windows 7(don't have a choice here, since Windows XP can only be run w/o ACPI and very unstable). Both 2 XP desktop join the same domain which the PDC is running in VMware on one of the desktop(I'll call this desk1). The PDC in use is a Linux distro call ClearOS(previously called ClarkConnect, side note: i have tried other distro like SME or eBox, SME just using a lots more resource while eBox just isn't as stable). Both desktop have setup share folders. While my laptop (Win7) isn't join the domain, the workgroup name is the same as the domain, and the username and password match the account on the domain. Right after I login to the laptop I will have access to all the shared folders on both desktop just fine. But after time have passed, and if I don't have any network file/folder opened, then I would lost access to desk2(which does not have PDC running in VM) and get the error message "You do not have permission to access \\computername\sharedfolder Contact your network administrator to request access.". I can still access to the shared folder on desk1 w/o any problem, and both desk1 and desk2 can access the shared folders of the other one without any problem. Right now I know that I could access that desk2 resource again if I logout and relog back in, then I would have access. And if I logon and leave a network drive or folder open, then I would not lose access.
I totally am lost on this one.