A software program made specifically for portable use is ClamWin
Portable, and you can easily find it online. I assume that it is a program that is meant to be downloaded onto a flash drive, and then it automatically runs whenever a person pops it into a computer. Then you don't have to browse to the drive, and start the program up. However, I think you can setup almost any program to run as soon as the flash drive is inserted into a computer. Simply right click the flash drive folder, and change it in the properties so it works that way.
Network administrators typically use a portable virus scanner so that they can walk around to each computer, and do this. I, however, don't think that this is the best way. Why not just run some remote desktop software, and scan from your computer in your office? That works just fine, and is much easier to do then running around trying to find computers.
My brother is a network administrator but I never hear him refering to his flash drive as a portable virus scanner. He uses his flash drive for many different things. In fact, his company has several different flash drives that are labelled for different uses. That way he can perform a virus scan on several computers at once without having to install anything on any of the computers. He likes doing it this way, and this is the way the company has been doing it for the past 10 years.
Of course, you can also use a flash drive for many other things such as holding movies that you can take with you and watch on other computers. You can use it as a transfer of files between one computer and the next if they are not in the same computer network. A lot of times people use flash-drives to store their pictures on if they have a digital camera. Then they upload them to their computer and begin using them very quickly. A flash drive is much more then just a portable virus scanner. You can use it for almost anything you can imagine, and you can even get it to open files as soon as you connect it to a computer. This makes it extremely convenient and efficient.