Bitdefender is a very good antivirus but it is true that it uses high system resources.
Avast is light on the system but i still think that you should try Bitdefender antivirus ones.
I have also used it and it is really very effective.
My firewall suggestion are Zone alarm ,Agnitum Outpost or Online Armor.

i am thinking for an Online armor premium license giveaway by me(win here on AVI's Birthday).
Can someone help me in Organizing this as i haven't hold any giveaway before anywhere.
Thanks in advance.