But...going to www.portableapps.com you can use their framework and make all your software portable!! Would be a great idea.
I am afraid things don't always work quite like that.
Many of the applications literally require themselves to be installed in program files because otherwise they do not have the access rights required to function correctly and app startup is handled by services which you couldn't install in a portable way.
Installation on a drive which could be removed would also be a very bad thing.