If you have trouble installing mcrypt on xampp on Windows, try this:

1. download "libmcrypt.dll" from http://files.edin.dk/php/win32/mcrypt/
2. copy "libmcrypt.dll" to your "C:\Windows\System32" folder
3. search the xampp directory for "php.ini", there are 3 files with this name. It would be smart to make backup of them before you edit any of them.

  \xampp\apache\bin\php.ini
  \xampp\php\php.ini
  \xampp\php\php4\php.ini

If you are not sure which one to edit, edit them all, for me the third one worked because I switched to php4 .
4. Find this line in the php.ini file:
;extension=php_mcrypt.dll
and delete the leading ;
5. Save-overwrite "php.ini"
6. Do not forget to restart your Apache server!

4. Find this line in the php.ini file:
;extension=php_mcrypt.dll
and delete the leading ;