All of the freebies are actually parts of full commercial 'security suites', with the manufacturers hoping to tempt users into paying for the 'bells and whistles' product. So, if you use any of them, you can expect to be invited to upgrade to the 'full' (i.e. paid-for) product when you first install the software and whenever major revisions to the antivirus program are required.
However AVG (which I use and currently seems to be the most popular freebie amongst ABers) isn't particularly intrusive. Every few months, when it's necessary to install the latest version of the program (because it can no longer be updated simply through the normal day-to-day background processes that go on all of the time) you'll be offered the chance to upgrade but it's hardly a problem - you simply have to choose between the 'free' and 'paid' versions when accepting the update. You might also see the occasional pop-up when you open your browser but you can simply click to close it. (It won't keep reappearing).
http://www.avg.com/gb-en/homepage