Simple; open the Start menu and click on the search option.
Type the autoexec.nt file and search your primary hard drive (usually C
.
Once you found the file then copy it to the right directory.
Don't move it, make sure you really
copy it. This is important because the file might be erased again in the future and you won't have any backup left if you simply click and drag the file instead of really copying it.
If you can't find the autoexec.nt file at all there is a link at the thread BeefontheBone mentioned which should allow you to find a viable copy from the net.