Windows 2003 Server network error

I am maintaining 500 clients running windows Xp Pro sp2 and Server running win 2003 Server enterprise edition .
My problem is I have created an OU and nearly 20 groups which consists of 40 users per group and in the user profile a folder is mounted to the user when they login, now iam having a problem that when a user types \\server\ all the folders which are shared on the server is accessible to them without asking for any passwords. Now I have to prevent users from accessing others folders. is there any options