I also have a zenbook ux21, and have encountered the same problem. It was indeed what people said about plugging in an external keyboard and pressing numlock, then disconnecting it.
If you don't have a keyboard readily available, there is still a way to fix the problem. I tried editing the registry, but it did not seem to work. What did work was writing a quick VB script to simulate sending a numlock press. Start notepad, and put this script in it:
set WshShell = CreateObject("WScript.Shell")
and save the file as FixNumlock.vbs
Run the script, and your numlock woes should be solved