antivirus on U3 drive

Nowadays, U3 flash drive is more and more popular.

Since then, there appears some antivirus software run on U3 platform, like avast! antivirus U3 edition, McAfee U3 Antivirus.

We know that in most cases, the virus infect a USB flash drive though copying an executive file and an "autorun.inf" from hard drive to the USB flash drive.

So can antivirus software on U3 platform protect U3 flash drive from viruses ?
And which one run first, the virus or the antivirus program ?

Thank you.

PS: After searching for some information, I found it seems that avast! antivirus U3 has an "On-Access" scanner and McAfee U3 antivirus only has an "On-Demand" scanner.

 

I have Avast running on my U3 drive. It works fine & you can use the U3 management to load the AV on startup. Avast will auto scan the attached pc & you can also "whitelist" trusted pcs that are not scanned.