You can use Symantec Ghost:
Easy to setup, but you have to create a bootable USB/CD/DVD to boot the system from. So if you want to install on 20 machines at once you have to create 20 bootable USB/CD/DVD.
You can setup a PXE/TFTP server:
Takes more to setup but you can boot every machine from the network.
You also want to make sure your network can handle that kind of load. I can install on 10 machines using our ghost setup in a little more than an hour. Our Ghost server and system stations are all connected up to a gig switch.