Apache / XP : Filesystem protection

Hi,
I'm running Apache2 under Windows XP SP2. And I also installed PHP5.
But for the moment PHP scripts can access any file on my hard drives. So i
have two questions :
- How can I forbid filesystem access to PHP scripts (and only allow certain
folders to be accessed) ?
- When I host several user websites : how can i prevent an user to access
the website contents of another user using a script ?

Thanks for your help.