Error 64 on VPN win xp to win xp

Hi,
I have a working VPN between two Win XP Pro PCs.

The client logs on OK and can see share files, but when it attempts to open
a file (in our example a Quickbooks file) the client gives an "error code 64".

This ref at microsoft.com: http://support.microsoft.com/kb/824864
does not mention error 64.
The only other reference in microsoft.com that I've found:
http://support.microsoft.com/kb/817069
says:
This problem occurs because the TCP window size for the TCP connection for
the VPN client is 0 (zero).

But it says it only applies to Windows 2000 !
and doesnt say whether the fix should be appied to the server or the client.

Can anyone assist me please - thanks
--
Thanks ... Barry