You can find all the installed programs on your computer, in Control Panel under Programs and features (for Windows 7) and Add/Remove Programs (for Windows XP). After you opened the above location, right click on the top bar (the one that contains Name, Publisher, etc) and make sure that the Location option is selected. By doing this, you will be able to see the location where all the programs from the list are installed. If you can't find the YouTube Downloader program there, it means that it isn't installed on your computer.