Looks like it's at
%userprofile%\Local Settings\Application Data\Tivo Desktop\TranscodingService.xml
in XP.
%userprofile% typically expands to
C:\Documents and Settings\[username]
in windows XP.
You can Run a CMD window to confirm that... CMD usually opens to that path by default, but you can also run "SET" and confirm that nothing has changed that environment variable (look for USERPROFILE= near the bottom of the list after running 'set').
I have not installed it on any of my Win7 machines, so I'm not sure about the location in that.