There's a free trial.

By all means stick to free but everyone I know who is infected has been
using free AV software.

--
Conor

The worst ****ed-up system I've seen as far as malware is concerned was a
system that was supposedly protected by a (fully updated!) and non-free
version of Norton Internet Security. (anti-virus and anti-spyware)

My nephew brought the system to me because he couldn't get it to connect to
the Internet at all using DSL. When it wouldn't connect to the Internet
through my cable modem either, I took it offline. Then I used my own
computer to download a couple of free AV programs and copied them to CD.
Used the CD to install the free AV programs (1 at a time) on my nephews
system. The first one I tried (AVG) found thousands of infected files of
dozens of different viruses and trojans. Note that this was BEFORE I
connected to the Internet to get the latest virus definitions. Meanwhile,
Norton was sitting in the system tray all fat dumb and happy and doing not a
damned thing about it.

So I left AVG installed, uninstalled Norton. Magically, the Internet
started working as soon as Norton was uninstalled. Later used avast! to
scan the system and found several more viruses.

Spent hours scanning that system with various freebie programs (spybot S &
D, adaware, various antivirus programs, online and off) before I was
reasonably certain that it was clean.

But with Norton installed, the whole ****ing hard drive was (pretty much)
just one big virus. -Dave