The portable version stores all the V settings and history on the portable drive (instead of the host PC), which allows you to take a registered copy of V wherever you go, and run it on any PC which is running a supported operating system (Windows XP/200x/Vista/7/8).
The portable version does not need to be installed. Simply extract the contents of the portable ZIP file (below) to a folder on the portable device and then run V.exe.
You will need to re-enter your registration code to register the portable version.
This means that you can run the portable version from a cloud folder (eg, a DropBox folder that is synced across multiple PCs).
Note that every time you run V from a cloud folder on a new PC, you will need to re-enter your registration code to register that PC.
As long as you are the only person running V from the cloud folder, you only need a single license, regardless of the number of PCs from which you run V.