I haven't read the other answers but to clean this off one of my schools I had to disconnect EVERTHING from the lan and make sure all servers were clean
On each machine, I ran the sophos root kit/conficker removal tool - if the machine had an infection i rebooted and ran it again and then did a scan with Malabytes malaware just ot be sure.
Once I'd done this - i reconnected machine to network.
All computers set to not autorun USB drives and all USB devices with memory in them e.g cameras as well as pens) were banned from school until I personally had checked them and then they were not allowed on school machines if they were taken home - they had to be handed back to me again for rescanning .
It took a week - i did the classroom teacher machines first.
Been clean for a year now :)