It sounds as though you previously had the Sage folder on one machine mapped to the drive letter "Z" on the other.
On the machine which has the problem, open My Network Places
Double-click Entire Network
Double-click Microsoft Windows Network
You should now see the name of the workgroup(s)
Double-click the workgroup
You should now see the two computers on your network\
Double-click on the one that you are NOPT sitting at
You should now see a list of the shared folders on the other machine, and hopefully one of them will be your Sage folder.
Now that you know its name,
Click on tools/map network drive.
The drive letter should default to Z:, if not, choose it from the dropdown list
Click the Browse button next to the "Folder" field
Browse to the folder where your Sage is installed, and click to highlight
Click OK
Make sure that "Reconnect at logon" is ticked
Click OK