Security

if you don't want an antivirus solution: never connect the system to any network and never copy anything on the system.

Or if you don't want this, keep the system up to date, keep firewall enabled, use common sense and install an antivirus (Windows defender is ok but not really good.

But the most important thing: think before do anything

windows defender is on by default but look at free AV software like:

http://free.avg.com/us-en/free-antivirus-download

What operating system?  What's it used for? Where is it used?  Do you use open wifi?

You might consider using a VPN if you use a lot of guest wifi, especially if it's unsecured. 

Don't turn it on.