For example just logging in and out tends to eat a little space so your best answer has got to be "try it and find out."
I'm taking bets that the answer will never be the same from my machine to your machine to Joe's machine.
I've googled, asked, and surfed for a simple answer, to no avail, so here I am again. DISREGARDING any additional software, ie limewire, canon, bla bla bla... how much space will my OS take up by just the addition of a second admin account? And.. if I remove one admin account later will vista hold alot of leftover "junk"? Is that whole account saved say in the recovery partition. You get my idea...right?