rommy_tqt, your going to get a million different answers, even security experts don't agree, but they do agree that its best to have an antivirus and at least a couple other scanners thats good for spyware/trojans.
I've tried the big 3 free virus scanners and they all worked great for me and most people consider them sufficient, comes down to personal preference.
AVG, AVAST, ANTIVIR.
If you trust a paid version more most people are recommending nod32 and kaspersky these days.