You could always copy system.mdw to your hard drive manually, i.e. from the Office CD.
(From memory, the Office CD is basically the same as what it should be on your hard drive, but with PFILES instead of "Program Files")
Since I have a few versions of Access installed, and I think it's also dependent on which version of Windows you've got installed, but you should have system.mdw in either:
C:\Documents and Settings\USERNAME\Application Data\Microsoft\Access\System.mdw
C:\Windows\System32\system.mdw (I think this one is for Acc97)
If, for some reason you ARE using Acc97, then you should be able to check what your workgroup is by using the Workgroup Administrator
However, Microsoft, in all their wisdom, decided to incorporate that into Access itself in subsequent versions.
If you've got an mdw file somewhere - anywhere, then you could always open Access in that workgroup, by using the switch /wrkgrp
msaccess.exe /wrkgrp C:\Some\Directory\file.mdw
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