Along with an AV and firewall, you can have a non-memory-resident tool like SpywareBlaster to block IE and FireFox based hijackers/spyware/adware.
http://www.javacoolsoftware.com/spywareblaster.html
And, if you are using Windows XP SP2 or later OSes, then you would most probably have Windows Defender. Windows Defender is quite good at monitoring critical areas in the system and intercept the changes (like driver/service registration, changes in critical areas of Registry etc.).
Regarding performance (memory/CPU usage) and detection rates, you can refer AV-Comparitives (
http://www.av-comparatives.org/ ). Check the "Performance" and "Main Tests" section in "Comparatives/Reviews" link in the main page of their website.