**Bash + Xdotool, 36 bytes**

    for((;;)){ xdotool key 66;sleep .5;}

Just execute it in a bash shell.  It needs to be in a graphical environment.  Infinite loop from [here](https://codegolf.stackexchange.com/questions/15279/tips-for-golfing-in-bash).  Changed Num_Lock to 66 to save 6 bytes, and thanks to @Michael Kjörling for 2 bytes.