I ran into that problem too. If "Run as Administrator" doesn't work (I don't think it did for me), re-install Workbench in "C:\Adobe\" NOT "C:\Program Files\Adobe\", which is the default. I think the Program Files folder has some kind of security setting on it that doesn't allow Workbench to add log entries.
Once I did that, I've been able to run Workbench on Vista with little or no issue (even though technically Adobe doesn't support Vista yet).
Hope this helps!
Ryan D. Lunka
Cardinal Solutions Group