Free antivirus applications get less signature updates and (mostly) lack good heuristics. The first point is the worse.
In that case I am glad I never purchased Avast as it must have been downloading updates constantly. It was not uncommon for the free version to download 3 or 4 signature updates a day.
IMHO all AV solutions are pretty crap as they are either too aggressive or rely on user common sense - neither of which leads to a good outcome - and I have yet to find any AV solution that stops everything ... that solution just doesn't exist.
I have certainly found more infected computers WITH paid for solutions than the free solutions - especially the big names who really don't seem to do that well, as far as I can see, stopping common webpage based attacks.
The reasons I now use MSE are:
1) It is free.
2) No advertising - quiet in use unless there is something that needs to be reported.
3) At least as good as any of the alternatives (paid or free) - and better at some of the webpage based crap ware that works its way onto systems than most of the others.
4) You can do manual; scans or schedule scans daily or weekly and limit system impact during those scans.
The only piece of software I think is even more snake-oily than AV is third party firewalls. If you are a techy kind of person they can be useful but for most people they are significantly worse than useless - which is why I advise anyone who has to ask the question to stick with Windows Firewall and a NAT router.