mcrypt is built in for PHP 5.3.x on windows, you don't need to do anything with a dll anymore.
As of PHP 5.0.0 you will need libmcrypt Version 2.5.6 or greater.
Windows users will find the library is the PHP 5.2 Windows binaries release. PHP 5.3 Windows binaries uses the static version of the MCrypt library, no DLL are needed.
If you linked against libmcrypt 2.4.x or higher, the following additional block algorithms are supported: CAST, LOKI97, RIJNDAEL, SAFERPLUS, SERPENT and the following stream ciphers: ENIGMA (crypt), PANAMA, RC4 and WAKE. With libmcrypt 2.4.x or higher another cipher mode is also available; 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:
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