Change the Windows 7 Taskbar Thumbnail and List Mode

Manually in Registry Editor

1. Open the Start Menu, then type regedit in the search boxand press Enter.

2. In regedit, navigate to the location below. (see screenshot below step 3A)

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband

3. To Change Thumbnail Previews Threshold Before Switching to List Mode

A) In the right pane, right click on a empty space and click on New and DWORD (32-bit) Value. (see screenshot below)

B) Type NumThumbnails and press enter. (see screenshot below)

C) In the right pane, double click on NumThumbnails. (see screenshot above)

D) Type in how many thumbail previews you want to have display so that it will switch to list mode after this number, then click on OK. (see screenshot below)
NOTE: Typing in 0 will result in always being in list mode.

E) Go to step 5.

4. To Restore Taskbar Thumbnail Previews Mode
NOTE: This is the default setting.

A) In the right pane, right click on NumThumbnails and click on Delete. (see screenshot below step 3B)

B) Click on Yes to confirm. (see screenshot below)

5. Close regedit.

6. Log off and log on, or restart the computer to apply the changes.