Computer Config login script from GPO copying file reports permission denied

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I created a VBscript login script under computer config on the default
domain policy. It checks the size and date last modified of a file on
the local computer (XP pro SP2) and a reference file on the server
(Server2003). If they are different it renames the local file and
copies the file from the server to replace it.

However, it always reports that permission is denied.

I presumed that the script would run in the security context of the
local system. And SYSTEM has full control permissions over the share,
the remote file and the local location.

Have I mis-understood the context that this GPO will run in?

Apologies if this should be in a vbscript group...

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