I installed PortableApps on my friend's hard drive. It works well!
My friend had no applications on his XP machine, so I installed Portable OpenOffice, Gimp, and several others in a folder I named "PortableApps" in his Program Files directory on drive C. I put shortcuts to the applications on his Quick Start toolbar.
Everything works fine, but Stellarium had to be installed in its own folder on drive C because it can't handle spaces in directory names.