Aero Peek is a new feature in Windows 7. If you have many windows open on your desktop, its quite easy to lose track of them.
If you wish to change the Aero Peek Desktop Preview times to make it faster or slower, type regedit in start search and hit enter to open Registry Editor.
Navigate to the following key:
Now right click in the right pane > select New > select DWORD (32-bit) value.
Name this DWORD as DesktopLivePreviewHoverTime.
Now double click it.
Select Decimal value and type the time in milliseconds. The default value is 1 s or 1000 ms. You may try 500 or 2000 and see what suits you best.
If you change your mind, simply delete this particular key and things will go back to default.
Alternatively, you can use Desktop Peek Tweak.