OK, you have a high speed line coming into your office (home?)that connects to a router. That router then connects to all of your PC's either straight from the router, or through a seperate hub/switch. Is that correct?
If it is, it depends on the router. At the very least, youare soing some Network Address Translation (NAT). In and of itself, this gives you some protection, but it is far from being fool proof.
Many of the small routers today come with some sort of firewall protection built in, but most require that you do something to make it effective.
Rather than speculate beyond this, check the documentation that came with the router and find out what it is capable of doing. Then make the changes that would be effective for your environment. You'll usually find teh documentation on a CD that came with the router. If not, go to the vendor's site and download athe manual for the model you have.