I like GravityZone a lot, it's got a very clean UI & I'm a big fan of the management console. I run it on my mother's & sister's machines too (with user actions on quarantine disabled because, well, they aren't very security minded & I don't trust them). It uses around 1GB of RAM and very little CPU if not actively running a full system scan, I haven't noticed any slowdowns or laggyness using it.
CyberLock is a relatively new addition to my setup, but it seems to be playing nice so far, I've not had any issues or conflicts.