It is never recommended to run more than one antivirus engine at once. I'd be willing to bet that I hammer on the internet more than your average user and I don't have problems with using just Microsoft Security Essentials along with their own Microsoft Defender.
The things that get most people in trouble, and I do this as a very poorly paying side gig
, is spending time on Torrent Sites and other file sharing systems such a Limewire. Using any of these types of sites is like playing russian roulette. Eventually you are going to get a nasty file disguised as "naked Britney" (or whatnot) or getting a cease-and-desist letter from the MPAA or RIAA via your ISP.
The other thing that gets a lot of people is those STUPID emails from your friends. We all have at least one person that sends out a dozen emails a day clogging our inboxes of jokes and links. I request to be taken off their lists and barring that their emails go to the permanent delete bin before I get a chance to even see it.