Macs now are targeted by malware writers too. Recently, for example, 600,000 Mac users were infected with a remote control trojan.
I concur with the vote for Linux.
For those infected, if you can't get rid of the infection, try switching your anti-virus to Avast (free for home users) and run a boot time scan. If that doesn't work, you may have to pull the drive and clean it from another system, or pick up an anti-virus boot disk. Some of these infections will survive even if you format the drive and reload the operating system. In those cases, the infection must be bit-wiped off the drive.
If you're in the western PA eastern Ohio area, I can help if needed.