I run ZoneAlarm on the computers on the network and keep my virus protection up to date bi-weekly.

Okay, that's a good start. But without a layer of NAT in between your computers and the rest of the internet, there's still crap that can slip through. Having the NAT layer lets you sleep at night, and take vacations once in a while.

If I dis-able the Hijack web and ftp features, will that mean that even friends that I have given the URL to won't be able to reach it?

Correct. But if you don't want it hacked at, you can't have it exposed to the internet anyway. I do not recommend setting it up as a public web server. Mark's features were only ever meant to be used on a local network, not on the public internet.