4.0.6 mssql.dll Extension load failure

We have PHP installed as an ISAP module on our W2K IIS5 server and its operational with no problems - phpinfo() runs just fine. But - When I uncomment the php_mssql.dll extension in php.ini I get an error when PHP starts up saying it cannot find the .dll file. The extension directory is correctly set to, c:\php\extensions and the actual file itself is in that directory as wells as c:\winnt\system32 and MSSQL client libraries are loaded and operational. Cant think of anything else I need to do - does the .dll file need be registered with regsrvr or something ?? Can anybody give me some pointers please ? Thanks & rgds Mike mikep45@hotmail.com