Vista - standard user can see admin files after UAC asked for password

I have vista business edition (up to date) and here's my problem; There's 2
standard user account and 1 administrator account.

While being on a standard user account, I entered the admin password to get
pass the User account control pop-up. However from now on, the standard
account now have full access to all files and folders for any account! How do
I fix this problem? I want every account being private.

PS: I tried changing admin password, no results. Only creating new standard
account fix the problem but it is annoying. Please help! Thank you.