It's reasonably self-contained. But I have my own doubts about the word "completely"
Not got the chance to play with a 64bit version, but generally portable apps comes down to (3 different points when it comes to complete portability
1) registry entrys
2) browser history
3) network access history
You can add system modifications as well...if it creates folders or leaves files around on a system ...such as a profile folder in appdata folders and such....then it is not really portable.
Most people assume a portable can be used on multiple platforms without having to worry about system modifications or registry entries that will result in errors later when it is no longer on the system.
A portable is also assumed to retain all its user settings within the portable's own file structure.
I have made a reasonable attempt at this if you would like to test it out it can be found Here
in the forums