Keep in mind that it is a an old game, and take note that no verson of Cossacks 2 works properly on 64bit operating systems (Vista and 7) with
the expection of the Steam one. it was not built for 64 bit OS. I think
the installer looks for the file path for Windows XP and 2000 etc to try
and find the game. As Vista and 7 use a different registry file paths
the installer can't detect the game and therefore thinks it is not
installled. Becuase of this I think that the problem can never be solved
unless you play it on Windows XP. I was personally greatfull that steam
gave one the works on windows 7!.
You might be out of luck on this issue, sorry.