Problem with Mcrypt on Xampp

[opazz]

Mcrypt is not working on Xampp? Is there libmcrypt.dll on Xampp after all?
I get this kind of error...

Fatal error: Call to undefined function mcrypt_decrypt() in (address to my php file where I use decrypt) on line 239

What on earth this means?

If there not.
Where on Xampp I should put that file?