I don't know, but...

Check out Steve Gibson's website (grc.com) and try the internet security tests he has. That will give you a good indication if you need additional protection. If it's an option for you, consider one of the hardware firewall/router/switches. You can get a 4-port version for around $100 and they are pretty easy to use. You could then get rid of internet connection sharing and you can operate the 2 systems independently.