I am not 100% certain, but if you install the program under the root (ie c:\Program Files) and not in a users program file list (ie c:\documents and setting\User\Start Meun\Program Files) then it should only take up the space of 1 program.
XP is a pig and takes a lot of space on its own, so the last thing you will want is to let it take up more.
My question to you is this: Is there really a need for Multiple User Accounts on your computer? You may just want to look into a folder locking program to protect those files you don't want others to see. There are plenty of free ones out there to try/ buy and that should serve the same purpose (other than setting i guess).
I have a wife and child at home who use the same desktop as i do (when i get off my laptop), and the secure folder program makes it so they can't get into/ mess with, stuff they a> shouldn't see, and b> shouldn't touch.:angel: