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1. does the divx video file need to reside on a shared resource that both PCs can access ?
Yes. And you need to make sure that the Xbox 360 and MCE PC access the resource using a UNC path (i.e. \\computer\share\filename). Because Transcode runs as a service it cannot access files using drive mappings.
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2. does transcode360 need to have the same account logged in on both PCs ? (to explain : I have a user account on my MCE box and a user account on the 2nd PC)
No, as long as both accounts can access the shared files.
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3. I'm not clear from this thread whether transcode360 works with mapped network drives
No, see 1

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4. When changing the registry key on my MCE machine to direct transcode360 to my 2nd PC does it only work with IP address or can I indicate "\\pc_name" ? (reason: I have a DHCP server providing IP so this IP address will change regularly)
You can use a computer name, but leave out the \\, just put pc_name in the registry key.
Finally, if you are running Windows Firewall on the PC that will do the transcoding, go the Exceptions tab of the firewall, click Add Port and add a rule allowing access on TCP port 1401.