Windows 7 ALT-TAB - How to turn off previewing the window

In Advanced System Settings (accessible from the System control panel), click the Advanced tab, then the settings button for Performance.

In the list of Visual Effects, two options affect this behavior:

• Enable Aero Peek
• Enable desktop composition

The first is responsible for the translucent "peek" behavior and the second for the ability for alt+tab to show what DWC (desktop window composition) is getting as the image to show for the program in question. Without desktop composition, you won't get the preview icons.

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
value AltTabSettings:
0 - preview
1 - "old-style" (icons only)

press Ctrl - Alt - Del, then Esc (go to logon window and return back to the desktop) to apply this setting

If you want to automate the settings for Aero Peek and Desktop Composition, you can tweak these registry settings:

HKEY_CURRENT_USER\Software\Microsoft\Windows\DWM\CompositionPolicy 
HKEY_CURRENT_USER\Software\Microsoft\Windows\DWM\EnableAeroPeek

For example, to disable Aero Peek:

REG ADD HKCU\Software\Microsoft\Windows\DWM /v EnableAeroPeek /t REG_DWORD /d 0 /f

If you tweak CompositionPolicy, you may need to run (as admin):

net stop uxsms
net start uxsms

For some weird reason, it seems Desktop Composition is enabled when CompositionPolicy is "0", and disabled when it is "1".

A few other relevant registry entries to check are

HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\VisualEffects
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\VisualEffects

However, the interaction between the "Perfomance Options" / "Visual Effects" window and the registry is quite mysterious and I never found a decent reference about it.