Answer: Foxhound's executable code is stored together with Foxhound data inside the Foxhound database, so "restoring the old build of Foxhound" is the same as "restoring the old Foxhound database".

The post-setup process creates a backup copy of your existing Foxhound database, and you can restore that copy by copying the foxhound.db and foxhound.log files as follows:

On Windows XP, copy the foxhound.db and foxhound.log files from:

C:\Documents and Settings\All Users\Application Data\RisingRoad\Foxhound1\backup\previous_build

to:

C:\Documents and Settings\All Users\Application Data\RisingRoad\Foxhound1

On Windows Vista or Windows 7, copy the foxhound.db and foxhound.log files from:

C:\ProgramData\RisingRoad\Foxhound1\backup\previous_build

to:

C:\ProgramData\RisingRoad\Foxhound1

See also...
How do I backup my Foxhound database?
What is the upgrade policy for installing new builds of Foxhound?