problem with PDO ODBC on Win2k3 server

I'm having a problem whereby PHP can't seem to load the php_pdo_odbc.dll 
extension. The server has PHP 5.2.8 installed to "C:\Program Files\PHP" 
and in the "C:\Program Files\PHP\ext" folder I have php_pdo.dll and 
php_pdo_odbc.dll. At the bottom of the only php.ini file on the server 
are the following lines:

[PHP_PDO]
extension=php_pdo.dll

[PHP_PDO_ODBC]
extension=php_pdo_odbc.dll

The extension directory is set to the following:

extension_dir ="C:\Program Files\PHP\ext"

The system event viewer logs the following event:

Application popup: Warning : PHP Startup: Unable to load dynamic library 
'C:\Program Files\PHP\ext\php_pdo_odbc.dll' - The specified module could 
not be found.

I have checked that the file is actually there, and I've also tried 
adding a trailing slash to the extension_dir directive and restarting 
IIS to see if that helped. It still can't load the ODBC module.

any ideas?