NOD32 is the best if you want a paid antivirus product. as for F-Secure, never tried it but I've heard good reviews on it for the most part so I'll have to try it sometime.
AVG free edition is what I use and it's really good and the resource usage is very low. Avira is also a pretty good product that's free too, except for the pop up dialog that comes up asking you to upgrade to the premium version. Avast is an alright free version, the realtime monitor is good but the interface isn't exactly easy to use.