ZoneAlarm has no role in sharing the internet connection.
Long time ago XP users used ICS built into the OS and we have settings if ICS was being utilized.
But this method was very biuggy and very un-reliable and not used anymore.
If your getting a mifi type of device those are generaly like a mini wifi router that 4-5 devices can connect to. That just looks like a router with NAT to the PC so no different than your old router.
If you use the Connectify software instead then we cant guarantee the firewall will work properly at all. You would have to get all support for issues between ZA and that software from the company that makes Connectify. ZA cant support it. So if that company cant support it you have to get a router.
You will be better off buying a new linksys router (not that expensive) than trying something your not familiar with nor we will support with ZA.