The file you download and run prompts you for a destination. If you don't remember what this destination was, it's most likely your Downloads or Desktop folder.
The default setting in the Windows installer also creates a shortcut for you on your Desktop, though be aware that you may have accidentally unticked the option to create a shortcut.
If you can't find it in either of those folders, download it again and look for the prompt that asks you to choose a directory to download it in. Choose a directory location that you'll remember easily, and once the download finishes you should see a Tor Browser folder there.