I suggest you check out SecurityNewsDaily.com then search Mac Antivirus. They have good reviews, Avast is not listed. Also the new trojan is called "Flashback" or "Flashfake", they(SND) call it "a drive-by download". So it's really really important to have the latest Mac security update on SnowLeopard or Lion.
I've had very good luck with Mac. I use either Chrome or Firefox, Safari has issues.
I used to code windows and they had holes big enough to dive a semi through
But then I stopped coding back at XP so I'm not sure what current windows code looks like.
Good Luck, I'm off to continue my work on TinyAeroTrailer
Ed