If your computer is working slowly, you might see a message saying that the theme is using most of its allowed memory. If this happens, you can change to the Windows 7 Basic theme or a high-contrast theme to improve performance.
If you change the theme to Windows 7 Basic, you will no longer get the full Aero experience. Aero is the premium visual experience in this version of Windows. It features a transparent glass design, window animations, Aero Flip 3D, and live window previews.
Open Personalization by clicking the Start button , and then clicking Control Panel. In the search box, type personalization, and then click Personalization.
Under Basic and High Contrast Themes, click Windows 7 Basic or one of the High Contrast themes.