You can't install the ATI drivers in your Parallels virtual machine because it uses a virtual graphics card, not your actual graphics chipset. Parallels has already installed any drivers needed for your virtual hardware. All you need to do is make sure that 3D graphics are enabled in Parallels' graphics settings and enable Aero in Windows.
If you're still having problems, you might try brosing the Parallels support forums for some tips.
That being said, while Aero in a virtual machine does work, I never use it because it seems to slow everything down. YMMV