i used avast trial and that was nice for a while but my final decision that i will stick with for a few months is:
Avira Antivir Personal Edition
Comodo Firewall Pro

Best thing is that they are free. I use avira just for the antivirus scans but i use comodo's firewall and real time protection too. I checked all my processes for comodo and avira and together they total only 17,000 k of my memory.