I use BitDefender AntiVirus 2013, not the full suite (because I'm not willing to part with Online Armor Free, which is a great firewall).
After installing it, my system became much faster! (I'd been using Prevx, MBAM, and Microsoft Security Essentials before), and I really love the mini-scans it does. (Instead of a full scan that slows your system down, it does very short scans when the CPU is idle).
It has also caught malware the others didn't, so I'm really pleased with it.