How to enable Vista Aero Glass Windows?

My Windows Vista Home Premium Aero Glass feature wasn’t enabled. But I am pretty sure that it is already on my computer system, I just don’t understand why it shows no effect when I switched on to windows, why it does not have a live screen shot when I hover my mouse to the start menu, and all of its features are not working.

This has been working when I first installed my operating system, but now I don’t know why it suddenly stopped.
I want to know the procedure on how to enable it again.
Please help.
