how safe is this computer

Avast Free Antivirus / Premium Security
1

If PC 1 does not surf the net but is connected to the network and once in a while plug in removable drive. How safe is this PC?

If PC 2 have network connection and surf the net but does not download any software or plug in any removeable drive. How safe is this PC?

Of course both PC have AV software and firewall

2

If you keep your AV, Windows updates and software up to date you are pretty good.

If you are using removable drives you should use McShield.