If you wonder why you are not able to set Windows 7 Aero theme there can be two possible reasons for this. First reason can be that you own a Windows 7 starter version or Windows Home basic version which does not have the Aero effect. The second reason can be a software issue with the Microsoft Windows 7 which Microsoft is aware of and its working on the solution.
Even if you own a Windows 7 version which is either Home Premium, Business or Ultimate version of Windows 7, you might get stuck in Windows 7 Basic theme and may not be able to switch back to Windows Aero there which is responsible for the visual effects. Microsoft describes Aero Effect as :
Aero visual hallmarks include subtle animations and translucent glass windows, which you can personalize. Pick from the included color palette, or create your own custom tints with the color mixer.Advertisement
The problem with Windows 7 is that if you switch between Windows 7 Aero theme and Basic theme several times (no fixed number quoted), it may not switch to the Windows 7 Aero there at all. It will get stuck with a “Please Wait” message.
The solution to this problem is being worked upon by Microsoft and they will probably come up with a Hotfix for the same very soon. Till the time the hotfix is released, there is a workaround suggested by Microsoft here.
Get Back Windows 7 Aero Theme
To get back to Windows 7 Aero theme, first ensure that you are running one of these three version of Windows 7 – Home premium, Business or Ultimate. If you are using Home Basic or Starter, it anyways does not support Aero effects.
To enable Windows 7 Aero theme, first switch from the existing Basic there to a “Windows Classic” or “High contrast” theme. Once this theme is successfully applied, this will also show “Please wait” message and it will finish applying this theme. After the classic / high contrast theme is successfully set, you can now switch to Windows 7 Aero theme.
source – [softpedia]