what is the best free anti virus to run on a macbook air using OS X 10.8.3

ClamXav

None. But, if you must use ClamXAV.

Helpful Links Regarding Malware Protection

An excellent link to read is Tom Reed's Mac Malware Guide.

Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.

See these Apple articles:

Mac OS X Snow Leopard and malware detection

OS X Lion- Protect your Mac from malware

OS X Mountain Lion- Protect your Mac from malware

About file quarantine in OS X

If you require anti-virus protection I recommend using ClamXav.