Why do you want to know? ffmpeg is often installed as part of another application. My Windows 7 system has sooooo many copies and different versions/releases of ffmpeg they are almost too many to list.
However, assuming that the 'ffmpeg' files have not been renamed (disguised) for some reason you could open a command prompt (Start>Accessories>Command Prompt), then select a disk, ensure you are at the top of the structure, and issue a command line search.
C: (or D: or whatever drive)
cd \
dir ffmpeg* /s
If your system is anything like mine the list will be long. You might find it necessary to redirect/capture the output for later viewing.