antivirus and encryption for usb harddrive

i have recently purchased a 120gb wd passport ext usb drive.all i want is a antivirus for my usb that prevents virus from entering my usb disk drive when i plug in to other comp without av. so that i can filter it out at the first possible step.
criterias
1) standalone and small (with or without ability of updates)
2) realtime scan on usb disk with autorun so that it gets activated from my usb each time i plug in and scans files what i put into usb from other comp without av
3) with or without possibility of data encyption with it ( or i can do with other softwares available)

if some thing is there like this well n good. if not atleast the second criteria must be there.

is there a possibility available? or any other recomendation from members will be seriously appreciated

 

does this offer a real time scan when ever i plug in??