What is the best way to monitor my multisite network for hacks or malware


I was wondering if someone could tell me the best service or way to monitor the WPMU network to protect it from hacks or malware. The last thing I want to happen is build a great network and have it get infected with malware or hacks. Any suggestions would be very appreciated. I am looking for both protection and notification 24/7 to monitor the whole network.

Tom George