Consider how many areas you are trying to protect...

...as an example, if you use Microsoft Outlook for your email, Defender and Security Essentials have troubles offering full protection. If you are installing programs from downloads off the Internet, you would be surprised how many have background software or sometimes can present threats and contain intentional or unintentional malicious malware, adware or virus packets..

Often you can find good advice that you should not use two different AV/Firewall programs in unison and I would back that up as a truth.

I have faithfully used AVG Security Suite for years and have the current 2012 version and have stayed clean even though I delve around with hip waders on, in areas where threats like to spawn. AVG offers full ranges of protection from online searches, Outlook protection on all emails, protection search ability for downloaded software, a link scanner that works very good to alert you NOT to go into a URL unless you absolutely want to prior to opening the internet page, an Anti-rootkit utility included along with family protocols to lock out unwanted sites from young children, and of course a very functional and effective firewall. It may sound as if I am preaching AVG software but that is just my personal preference. There are many reliable utilities out there as others have mentioned.

You can often find free cut down versions of many AV/Firewall applications along with free Adware/Malware and they often work very well. Yet for a feeling of security I would advise getting the full versions/professional and having the emotional comfort of a tougher program to handle tougher environs out there in Cyber World.