Open the system services dialog like this:
Start | Settings | Control Panel | Administrative tools | Services
Locate the service called "MGAFGEXE" and double-click on it. Set "Startup type" to "disabled", and then manually stop the service.
WARNING: If the replacement fan fails, the card will overheat and possibly take permanent damage. If possible, use an original Matrox fan. We do NOT recommend you try to fix your card yourself unless you know what you're doing and are willing to take the risk!