MacinSpot

It is important to have antivirus on your new Mac and let me explain why. Despite the many issues with PCs, people still use them. A Mac can become a host to a virus just as easily as a PC. If you get an email with a virus attached to it and you end up forwarding that message to someone with a Windows computer, you have just sent that virus along with it. Aside from that Macs can get viruses too.
There certainly aren’t as many viruses out there for Macs as there are for Windows based computers, but they still exist. See this link below:
MacNN article about first bot net virus for Macs.
There are a number of antivirus applications for Macs. Avast is a good one, but can slow down your computer considerably. iAntivirus is my personal favorite. You can install the free version from here:
iAntivirus Free
…or if you don’t feel comfortable installing applications, you can call us and we will install the more professional version remotely which is $49.99 plus 30 minutes of remote time. With that comes support from us whenever you need it. Your Mac must be Intel based and running at least version 10.5(Leopard) of the Mac OS. Businesses are not permitted to use the free version of iAntivirus.
If you have version 10.4(Tiger) then you can download the free antivirus application ClamAV for Mac here:
ClamAV for Mac download

I hope this helps!