Well, most often thanks to word-of-mouth. It goes something like this:
Step 1: The need
A PC user with an expiring or troublesome antivirus figures out he has a need for new security. (This does not happen to Mac users because of course they DO know Macs don’t need any AV… ;)
Step 2: The call
A PC user calls his favorite geek or IT friend who knows EVERYTHING about computers. Yes, a PC user could also make the effort and learn about it himself, read some reviews, check some comparative tests, and so on, but that takes too much time.
Step 3: The advice
“get avast free antivirus …” OR “download avast free” OR “install avast home version”. Then your friendly geek hangs up because his time is valuable.
Step 4: The what?
The PC user is not really sure what it was he needs to do or where to go or how to spell it, but for this we have Google. So, he googles it out. Voila! Easy!
Step 5: Download
Google lists out the search results, the PC user goes to any of the top links, clicks-through to AVAST page and downloads avast! Free Antivirus. Some 15 million people do this every month, and right after they would choose install>accept>next>next>finish>thank you.