Hey there chris-f, welcome to Geeks To Go.
Well you could use your flash card as a 2Gig external drive, this means you can load any browser and anti-virus on the flash drive. Just when you installing the browser (Firefox) and Anti-Virus (AVG, Norton, ZoneAlarm, ect.) you must change the install directory to the flash drive instead your normal system drive.
The negative thing about having your anti-virus on the flash, it can only be used on your computer (The system you loaded it onto). 2nd is that if you forget to plug your flash card into your computer at startup, you will get all kind of errors from windows telling it cant find the program.
I guess if you have a browser on the flash it will work at any computer and you dont always have to plug the flash in at startup (Recommend Firefox). But I would recommend NOT puting on a anti-virus. You could think of using Ad-Aware and Spybot Search And Destroy on your flash card instead of anti-virus as this doesn't need drivers and files outside its folder to run. Although, you can try installing AVG on to the flash, but I cannot be 100% of its going to work the way it should. If you need any more information, let me know.