You can protect your computer very well with free programs.
You should however not rely on just one program.
Here are some very good programs, all for free and together they will protect your computer very well indeed.
Anti virus and firewall pro
http://www.personalfirewall.comodo.com/download_firewall.html
or
http://www.avast.com/eng/avast_4_home.html
in combination with
http://www.pctools.com/firewall/
Threatfire is an anti virus, that can be installed next to any other anti virus program on the same computer
http://www.threatfire.com/
This is a very good program, that completely runs in the background
http://www.comodo.com/boclean/boclean.html
Anti spyware
http://www.spywareterminator.com/
http://www.malwarebytes.org/rogueremover.php