An important note:
McAfee isn't just a single program. It's a suite of programs. If you're going to replace it with some of the free stuff you'll need to replace several different components.
Decent computer security requires:
(a) a good firewall ;
(b) an anti-virus program ; and
(c) anti-malware software.
Windows (from XP onwards) has its own firewall but it's widely criticised. The most popular free alternative is ZoneAlarm
http://www.zonealarm....ity-free-firewall.htm
with Comodo also having many fans
http://personalfirewa...om/free-download.html
AVG used to be the leading free anti-virus program but some people (including me) have experienced problems with the most recent versions hugging resources. Avast now seems to be moving ahead in the popularity stakes.
The king of the anti-malware programs is Malwarebytes
http://www.malwarebyt...cts/malwarebytes_free
The manufacturers of those programs give away the relevant parts of a full security suite in the hope that you'll buy the entire package. (Which is why you've encountered both a free and a commercial version of Avast). The trick is to to put a collection of freebies together, from different sources, to make you own complete suite.
Microsoft belatedly decided to produce their own security software, in the form of Microsoft Security Essentials.
http://www.microsoft....sentials/default.aspx
It runs on most PCs but a few older systems aren't compatible
http://www.microsoft....stemRequirements.aspx
It also requires the user to have a genuine (non-pirated) version of Windows, which probably rules out about half of home users in the UK anyway!
Chris