Need fast, reliable, working Mac security software!

My uncle has had his Mac for a while, and recently he tried to install something dodgy to watch a live football match, from his Mac. However, he nearly always gets ad pop-ups now, and he's had enough. I installed Ad-Block for him, but that probably won't be enough. What's a good virus scanner for Mac, that's free, and works quickly? It needs to work in depth, but not take too long. I was thinking avast! might work, but it takes a little too long. Any other suggestions or would avast! be the best?

 

The browser was Chrome, and I did check the extensions, but there was nothing dodgy there. I ended up sticking with avast!, and it found over 170 infections, all of which have since been deleted. I'm hoping that will have done it for hime, although there were a load of files that couldn't be scanned (presumably because they were core files). Nevertheless, it all seemed good.

On the other hand, the ClamXav thing seemed useless, and was full of warning on how the computer should be backed up first, etc. and it then had the nerve to say it needed to install another piece of software, at which point it was sent straight to the Trash.