Aero Peak lets you ‘peak’ behind all the windows so that you can have a look at your desktop, should you wish to access any gadgets or shortcuts. It does so by making all the open windows go transparent. To do so, you simply have to move your cursor to the extreme right part of your taskbar and hold it there. Or else click Win+Space.  But if you dont like or use this Aero Peek Feature in Windows 7, you can easily disable it.

To do so, move your cursor to the bottom right corner and right click. Here uncheck the option Peek at Desktop.

Alternatively, right click on Windows 7 Taskbar and select Properties.

Uncheck the option Use Aero Peek to preview the desktop.

Click Apply > OK.

You can also disable Aero Peek via Control Panle > Performance > Visual Effects, if you wish.

Thats it!

Go here if Aero Peek is not working in Windows.