You can uninstall Alienware Evolution using Add or Remove Programs feature:
If you use Windows XP or bellow you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Alienware Evolution program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Alienware Evolution application and click on Uninstall.
After that, remove manually all Alienware Evolution files from C:\Windows\Resources\Themes.
If this solution doesn't work, you can force it to uninstall using Revo Uninstaller, IObit Uninstaller or Advanced Uninstaller PRO. All of these utilities are free of charge and easy to use.