Why do I have to be an administrator to install a program?

In window xp no need of administrator but in window 7 for security purpose Administrator right needed.

only admin has the right

Well, only admin have the authority to make changes in windows operating system, like installing new program and deleting any program.. So it is not possible if we haven't logged in via admin or if admin haven't given special permission to any other user account.

Administration access is needed only in Window 7 Operating system not in Window Xp.