I would suggest
1. Avast Antivirus,
2. Spybot Search & Destroy alongwith Defender
3. Windows Inbuilt Firewall
4. And yes, lots of common sense while surfing as said by The_Devil_Himself
THE MOST IMPORTANT THING IS TO KEEP THESE UPDATED.
I am using these without any problem since months.
I have tried Ashampoo, Comodo and Zonealarm firewalls, they all gave problems in either first run or sometime later. Internet performance suffered in my case. So i dont prefer any of them. Windows Firewall is good enough, atleast for me. Its light on system and it works
ADAware 2007 has many bugs in its UI-Engine integration according to my testing and i would not like to use it untill they give a fix.
Cheers!