"True" portability vs Windows Prefetch?
I'm on a Mac at the moment, so I can't test this myself, but it just occurred to me...
As far as I know, every .exe launched in WinXP (and later) will result in a corresponding entry in the WINDOWS\Prefetch folder. This file will certainly indicate the date & time the .exe was active, and presumably has some encoded information regarding the .exe itself - its location on disk, and which disk, is probably stored but may not be readily legible to the human eye.
My point is this: Many people here go to great lengths to point out that some apps (notably TrueCrypt) are not "Truly portable", because they leave some traces. Also people seem keen to have security-related apps, sometimes including unerase tools, secure deletion tools and apps to clean up traces of their activity on the host PC.