When I install my epson cx5500 scanner it always ask a administrator... why? Please help me... :(

Windows and other operating systems have different types of accounts for different classes of users. Accounts that are members of the Administrators group have the most privileges and can install software like drivers and access all files on the computer. If you are getting a prompt for Administrator rights during software installation, the account you logged into the computer with and are using is not a member of the Administrators group. Account group membership may be permanently adjusted by using the Control Panel User Accounts applet or the Computer Management console in the Control Panel Administrative Tools. Both will require using an admin account to make changes to your account however using an admin account for everyday use carries a security risk as software executes with the privileges of the user.

What version of Windows are you using? Recent version of Windows have a User Account Control component that prompts for admin credentials (username and password) when a non-admin user attempts to perform a task requiring admin privileges.