How do i set my security for comcast internet in linux

http://free.avg.com/us-en/download.prd-alf

You don't really need security software. In order for an application to execute as root, it needs you to enter your password. As long as you don't randomly enter your password without reading what it's for, you're fairly safe.

I'll add that if you're really paranoid, close off port forwarding where not needed if behind a firewall, or install an easy-to-use firewall and do likewise if you're directly connected (firestarter is a good one)