Installing the php_mhash.dll extension

I need to install the php_mhash.dll extension to use with Authroize.net on my windows 2000 server. I edited the php.ini file by adding the correct directory and removing the ; before php_mhash.dll. I then copied the libmhash.dll file that is needed to the C:\Winnt\system32\ folder. I ran the test scripts and all worked well. However after I run the script it seems to freeze up other php files on the server. I try to go to a another php page and it doesn't load. I removed these 2 files and put back the php.ini to the way it was before and everything goes back to normal. Do I need to register the libmhash.dll in the command prompt or fix some configuration in windows to stop it from freezing my php files. I have never set one of these extensions up before.