A portable firefox is one thing, you might acutually need that, but this is something special in the case you can't install software on the machine.
In a case of a big game it should actually to do it the other way around: Install the game on different machines and put your /home directory (or how that is called in Windows) on a portable storage.
If you need to put PS on a portable disk because you can't install it on some computers, then you might just do both:
Put you userfiles on the portable disk
and put the game also there (in another directory).
On Linux this works really good, should also be possible on Vista, but for XP you need some special software to do a good linking to certain parts of your home folder.
On Windows you can also move your whole user directory to another harddrive (even a portable one I think).
Hope that helps.
EDIT:
Thinking of a portable game is also quite weird for a 350 MB installation hat uses lots of ressources. Portable firefox is quite a bit smaller *g*.
Thinking of portable firefox: It is kind of another branch. I don't know exactly if you are allowed to make small changes to the client like that and still connect to laanx and use the PS art.