Your obvious choice for price is going to be linux based.

You may want to look at some all in one solutions such as smoothwall or ip cop .

I can't get to ip cop at the moment...

You will need 2 nics in the box and it will run on older hardware too. Other than that... after you configure it, you can pull the monitor, mouse and keyboard and do remote administration.

Or, start building a linux based solution from the ground up.

Smoothwall or ip cop would just be easier, IMO.

Don't know about your virus protection question though. For home, I've always used everything on the clients. At work, I have a AV server, but it pushes the defs out to the clients. Not a practical solution for a home setup becuase of the co$t of the solution.