How do you install a game on only one account on Vista?

Whenever I install a game on the computer (It's vista) the game installs on every user account. It's annoying cause like my sister installs things on her account which automatically get installed on my account too, which I don't want. And vice versa. Is there any settings to change which accounts you install games on?How do you install a game on only one account on Vista?i think not but even if you cant see the installed files there still there taking up space.unless you want it to install files that your sister must not see?