Everybody should have antivirus prog installed, even if they have OS X.
It is the ignorance (or perhaps arrogance) of Mac (and Linux) users that keeps viruses in the wild.
Even if your system can not be infected, you should contribute to keeping things clean everywhere. Install now.
If you think Avast is not good, try something else. But do install antivirus.
Have OS X with Avast, have Win XP/Vista/7 with Avast, have Android with Avast.