What is it?
You don't need to spend a penny to be adequately protected. It's all free. Windows Defender is OK for real time protection, Ad-Aware, a-squared, Spybot Search & Destroy for scanning occasionally, SpywareBlaster to protect you from dodgy sites...and yer done.
Provided, of-course, you have a decent antivirus (Avast or AVG) and firewall (ZoneAlarm).