The (in)famous 000.exe virus.
This is the full source code for 000.exe which was used and shown in the making-of video (https://www.youtube.com/watch?v=e_TYnADDsLQ). The "Initial commit" state is the exact state as displayed in the aforementioned video.
If you want to run this yourself, you first need to download the file "globalKeyboardHook.cs" from https://www.codeproject.com/Articles/19004/A-Simple-C-Global-Low-Level-Keyboard-Hook. Since the file is licensed under the CPOL v2.0, it is not compatible with the GNU GPL v3.0 (source: https://www.gnu.org/licenses/license-list.en.html#cpol). If you want to build this yourself, go and grab the file and place it in the root folder of the project.