I can vouch for Avast free version as I use it on all of my PCs and Android tablets. Avast. Note on installing use the custom install so you won't download a toolbar offered (unticking the boxes). You will also need to register for a year's free protection and register every year.
Some people use others such as the free version of AVG.
Which ever free AV you settle on you should follow McAfee's instructions to fully uninstall it first icluding running the MCPR tool , McAfee removal.
"We all have our favourites. Mine is the free version of AVG."
IMHO it's always best to check the results from independent AV testing companies when deciding on a such an important thing as computer security. AVG Free always seems to do consistently well - here's the latest report from AV-Test.Org.
" will agree to disagree with you on that I think it (MSE) is fairly poor but it is free."
I think you may be right there as there have been some very worrying press reports about MSE's effectiveness recently - have a look here and here.