http://free.grisoft.com/doc/5390/lng/us/tpl/v5#avg-free
For good reference, ask other members about this. I use AVG and ZoneAlarm, but I'm low on memory, so I'm sure there are way better ones. All of those are for Windows, mine you. If you want a simple and easy one, click on the very first one and install it. It's for Windows. If you don't have Windows, scroll down and there'll be other types too.