Unzip Avisynth and copy AviSynth.dll to the "Windows/system" directory.
Doppelclick on "install.reg" and reboot.
To test if the installation was successful create a file called "test.avs".
Open this file and modify it as stated below.
Save it, and open the Media Player.
Now open the "test.avs" in the MediaPlayer.
The MediaPlayer should look like this now.
That was all.