Setting some security issue is always a tough task. Hackers / crackers have their own unique ways of getting through, in an unpredictable date or way. They use different names just to deceive / fool the secured area. Some firewall get immune they treat some attacks as a healthy one, soon to discover that is it destroying the whole system, same as the network. And when the network is infected, the whole operation shuts down, thus, force the administration to come up with a quick decision.
Reformatting is the quickest way to erase the known attackers and to start a fresh system which includes the whole network. This is not a best decision, it will cost a lot, and the data that has been kept for a long period of time may lose in a just one click away.
Antivirus cleaning may come in hand but it is not 100% perfect and that antivirus will get immune too. It will also cost a lot; infected data will be put into antivirus volt, which is still a part of the whole system. Some infected may be deleted but hope that it is also deleted in the systems registry.
If I am the admin, then I have to choose on reformatting. Set the firewall and antivirus first before installing any fresh copy of application of copying any files on the system. Though time will come for the new kind of virus to infiltrate the system, still, it will take time. And for the currently known virus, then its going to tough or no chance for them at all.
Set your personal firewall and consider those data who you want to protect and kept forever.
