The built in firewall for windows 7 is pretty competent enough.. That along with windows defender should provide you adequate security.. The rest depends on how smart you are with your downloads and other internet related stuff.. If you still need extra security then trying using just an anti-virus software along with the windows firewall.. I'll recommend Nortan Anti-virus 2010 or windows security essentials for that purpose.. Both of them are light on resources and good with security..