Totally agree with last post! Connecting to the Internet without up to date Antivirus software and also Spyware software is like Sleeping around without protection.... Only a matter of time... Ace while it lasts!
The best way to connect 2 PC's to a cable modem would have to be the router.
1. If your smart and get a router with a firewall as mentioned before, you can rely on internet security.
2. In the case of a network sharing the modem, you'd have to turn on the host PC to get internet on the other, a router is always on with the modem and cuts this annoying problem out.
As for viruses, unfortunately they travel by networks of all shapes and sizes. So any PC you attach to a network is fairly vulnerable. So as mentioned before get antivirus software on each machine. (I use trend microtech online, basically because its free and the pattern file for detecting the little blighters is always up to date).
But back on the bright side of things, routers are easy to set up and maintain. The configuration is usually in the shape of a webpage (instructions tell you the address to point Internet Explorer to) and from there you can open certain ports for whatever purposes you see fit. And as for the two computers connected to the router, there should be no firewall between them (only between the internet and the rest of the router)
If I have convinced you or you need any more help, then shout out