Good evening Bobh
Most definitely maleware bytes does offer a free version, but the paid version does have some perks to it. But you cant go wrong with the free version even the free version has help my customers and a lot of end users countless times.
#1. Free version of MWB you don't get boot time scanning, you cant schedule scans and no support other than FAQ and googling for fixes etc
#2. Keep one thing in mind you cant have 2 antivirus's running at the same time. (active / running scanners - inactive / manual ones are different) If you run more than 1 active AV suite at a time you will have issues. ie BSOD, freezing or over all system performance etc.
#3. Great free AV is Avast, again free so you do loose some perks but you do get the ability to schedule scans, remote desktop feature, firewall, psh'ing filter etc.
#4. But if you want a great and CHEAP AV suite that I use my self and is the only paid AV suite that i offer to my Clients is "Vipre 2012 suite" they do have just a AV but really for a couple of buck more (i do mean under 10 difference) you can not beat features, cost per lic and per year and ill not slow a PC up like Norton, McAfee, Trendmicro etc.
There tech support is second to none, HERE in the US, friendly and quick. Plus you can under stand what they are saying because they speak in English and no techy terms are used, oh ya the phone tech support is FREE for as long as you have a current lic.
If you need any further assistance please let me know, Mike