To play N64 games on a computer, you must first download an emulator to run the games on your computer. To download the emulator, go to http://www.coolrom.com/. Browse emulators until you find emulators for the n64. From here you can download one of four emulators. It dosen't matter which one, they all run most games, however i do recommend project 64. Be sure to save the emulator files to your flash drive, not only the shortcut.
Side note: At this point, you can download emulators to other game systems if you wish. The process is practically the same.
Edit: I forgot to add that you will need to install the emulator onto your flash drive also. This takes about a minute. After downloading the emulator onto your flash drive, you will see a shortcut for project 64. Click on it and make sure to install the file onto your flash drive. Several files should appear after the installation. To run the emulator, click on the red shortcut.