After a while I could figure out how to install the Mcrypt library on Appserv and windows.

Steps:

1. Remove the ; (enable) from the php Mcrypt extension line on PHP INI file
2. Copy the windows Mcrypt DLL - http://files.edin.dk/php/win32/mcrypt/ - to Windows/System32 folder
3. Restart Apache (and probably windows - I did)
4. Open a phpinfo file and verify if mcrypt is enabled and what types of encryptions does it carries. If you are using a encryption method that mcrypt does not offer support, it will give a mcrypt module initialization error.