mcrypt is built in for PHP 5.3.x on windows, you don't need to do anything with a dll anymore.
Requerimientos
Estas funciones trabajan utilizando » mcrypt. Para usarla, descargue libmcrypt-x.x.tar.gz desde » http://mcrypt.sourceforge.net/ y siga las instrucciones de instalación incluídas.
A partir de PHP 5.0.0 necesitará libmcrypt Versión 2.5.6 o superior.
Usuarios de Windows encontrarán la biblioteca en los binarios de Windows de PHP 5.2. Los binarios de PHP 5.3 para Windows utilizan la versión estática de la biblioteca Mcrypt, no se necesita de ningún DLL adicional.
Si lo enlaza con libmcrypt 2.4.x o superiores, las siguientes bloques de algoritmos adicionales serán soportados: CAST, LOKI97, RIJNDAEL, SAFERPLUS, SERPENT y los siguientes cifradores de flujo: ENIGMA (crypt), PANAMA, RC4 y WAKE. Con libmcrypt 2.4.x o superiores, otro modo de cifrado está también disponible: nOFB.
Both mcrypt and libmcrypt are available through the linked website. You simply have to click the "Browse all files" or similar link once on the "development site" and not just look for the default Sourceforge download button.
If installing libmcrypt from RPM, you need both libmcrypt*.rpm and libmcrypt-devel*.rpm. I found RPMs for both were available for most platforms on this page:
http://rpmforge.net/user/packages/libmcrypt/
Also files.edin.dk does not seem to carry the win32 files anymore.
I found the libmcrypt-2.5.7.tar.gz file here: ftp://mcrypt.hellug.gr/pub/crypto/mcrypt/libmcrypt
The mcrypt link above only has mcrypt-2.x.x.tar.gz files which when compiled said it was missing the libmcrypt file. The libmcrypt-2.5.7.tar.gz worked when compiled and installed on Fedora. I had to use configure options --disable-posix-threads --enable-dynamic-loading for libmcrypt, and -with-mcrypt=usr/local/include configure option for php, which is the folder with mcrypt.h
