20% of corporate computers have malware

Yes, they run antivirus. No, it doesn't help:

A study by Damballa demonstrated that the typical gap between malware release and detection/remediation using antivirus is 54 days. The study was comprised of over 200,000 malware samples scanned by a leading industry antivirus tool over six months. The study also revealed that:

• Almost half of the 200,000 malware samples were not detected on the day they were received

• 15% of the samples remained undetected after 180 days

Damballa is taking an interesting approach, focusing on the process where malware "phones home" (the Command and Control channel). Pretty clearly, looking in files like antivirus does works pretty badly.

It's clear that anivirus is now the equivalent of the lock on your door. Of course you lock up at night before you go to bed. It will even keep casual intruders out. But it won't stop - or even slow down, really - a knowledgeable and determined adversary.

This is the biggest reason to suggest changing from Windows to Linux. You don't get coverage of a lot of games, and you lose iTunes. However, you simply will not need antivirus.