In case if you have an old computer and want to use normally Windows 7 you probably need to disable Windows Aero Glass interface for Windows 7. There are a lot of ways you can disable it in windows, the easiest way is the next:
1. Right-click on Taskbar and select "Properties".

2. Uncheck "Use Aero Peek to preview the desktop" and click on "Apply".
After that Aero effect will be disabled and your overall computer performance will increase.