ASMX runs in IIS 6 but not IIS 8

Hi Guys, 

I'm trying to migrate my applications/web services from an old server with IIS 6 to a new server running IIS 8

However it doesn't work in IIS 8.. After entering the required parameters and click on "Invoke" button, "The page cannot be displayed because an internal server error has occurred." appears.

I tried debugging and found that there isn't any problem with the web service but the code inside. The web methods are trying to encrypt and decrypt using - MD5CryptoServiceProvider

Returning a dummy blank string works but when i add this line - MD5CryptoServiceProvider xx = new MD5CryptoServiceProvider(); It returns - The page cannot be displayed because an internal server error has occurred. above invoking the method.

Any help is very much appreciated.