Vista - WMI ExecQuery from Win32_NTEventLogFile in a Workgroup environment

Scenario:
- a number of PCs running Windows XP Pro
- all have "Domain" set to "Workgroup" (windows installation default)
- all are logged in as Windows default "Administrator" account
- none of the nodes have Windows Firewall enabled

I have a script that runs on one node and copies files from all the remote
PCs into the folder that the script is run from. This part works fine,
gathering files from both the local and remote nodes, and placing them in
this folder.

I want, however, the gathered files to also include the Windows Event Viewer
..evt exports from all the remote nodes. Here's what I'm using for this:

Set objWMIService = GetObject("winmgmts:" _
& "{impersonationLevel=impersonate,(Backup)}!\\" & _
strComputer & "\root\cimv2")
Set colLogFiles = objWMIService.ExecQuery _
("Select * from Win32_NTEventLogFile " _
& "Where LogFileName='System'")
For Each objLogfile in colLogFiles
errBackupLog = objLogFile.BackupEventLog( _
dest & "el-sys-" & node & ".evt")
'WScript.Echo "File saved as c:\scripts\applications.evt"
Next

This works fine for the local node from which the script is being run,
putting its .evt data into the folder. It does not however gather .evt data
from any of the remote nodes across the network.

From the reading I've done, I gather that the problem may relate to being on
a Workgroup vs. a Domain. I've also seen mentioned that it may be getting
forced into a guest login, but I've confirmed that all nodes have this
setting:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
"forceguest"=dword:00000000


I would appreciate any ideas as to why this is not working for remote nodes.

Thanks,

David