I am playing the GOG release (which uses ScummVM) on a Win7 64bit machine.
I downloaded it via Firefox. [I don't use GOG Galaxy.]
My downloads (from any source) automatically go to a folder named Downloads on my D Drive.
Once the setup files for Lighthouse had downloaded to my D Drive, I created a folder called GOG GAMES on my C Drive and moved the files there.
I installed the game to C Drive.
My route to my saves is :
[I use both C and D drives for my games as both are very big drives and afford me plenty of space.]
Time : The Most Precious Commodity