I used to be a big advocate of the free antiviruses until a couple of years ago.  There was a plague of infections and almost every single system I cleaned up had a free antivirus on it.  Anecdotal, yes, but there it is.

Since then, it's become an unfortunate standard for free AVs to do things like

• change your default search engine to their "Safe search" (which may be ad-supported and may lead you to products on which they get commissions)
• change your homepage, again to a page that may lead to them making advertising dollars
• add toolbars to your PC.  Argh!
• bundle garbage software to your system to "enhance" and "optimize".  They get kickbacks from the software authors.
• track your surfing habits and sell that information to other companies.
• bundle in their "tech support" software for which you might get charged a good sum for using their services.   Comodo's Free AV does this.

This is not to say that the paid versions of these AVs are not worth it as, at that point, you've already paid for the product so they shouldn't be pushing garbage on to you.  Regardless, I always recommend paying attention to ANY software's installer to make sure it only installs what you want and not extra junk.

Full article on HowToGeek.com