If you installed Windows Vista, 7, or 8 in VirtualBox, you probably noticed that the Windows Aero theme is not enabled, even after having installed Guest Additions. This is because there is a trick that you must do when installing Guest Additions in order to enable it.
When installing Guest Additions, you'll want to check the box that says Direct 3D support. At the message box that comes up next, click NO.
Keep in mind that Direct 3D support in VirtualBox is experimental, so some applications may not work properly. For example, Google Chrome did not work correctly for me after installing it. However, it is pretty stable, so most applications should work just fine.