Hi, and WELCOME TO TECHIMO - from the Folding@Home Team [#111]..!
It sounds like there was a shared folder on computer "C" that you haven't re-created; haven't created the share with the same name; haven't called the computer the same thing; or haven't allowed the same level of permissions to it.
From what you've said, or rather haven't said, I would imagine that isn't going to be the last of the options I've stated, otherwise you would've been presented with a prompt for credentials and you would be able to get past that.
Looking back through it again, it sounds like the computer was originally called "Sue_cfs" (or SUE_CFS, as computer names should be 'written' in CAPS) and the shared folder appears to have been called "Z".
I would go onto the "C" computer and check what name it is using, if it doesn't match with the original name, then you can either amend the name to match the original, or just create a new drive mapping to whatever the new machine name is. I would also right click "My Computer" (in XP, "Computer" in Fista or W7) and select "Manage" and go to the "System Tools" --> "Shared Folders" --> "Shares" to see what folders are shared out.
If the original folder location still exists but isn't listed as being shared, then you can right click the folder and select the "Share" tab in the Folder Properties dialogue box (or just use the Computer Management window you just opened, there's a 'wizard' in there) and just make sure you allow the share permission to the "Everyone" group to have full access in the "Share Permissions" section, and lock it down on the "Security" one instead......
Let us know how you get on..!