Recently, my Windows Explorer under XP stopped showing thumbnails of certain movie files. I believe this may have happened during the installation of VLC. In any case, you will need to hack the registry to remind Windows Explorer how to display these thumbnails. Copy the following code into a file called fix.reg, save the buffer and double click on the file. Restart Windows and try to forget this ever happened.

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.asf\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.avi\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.mkv\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.mpeg\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.mpe\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.mp4\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.mov\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.rm\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.rmvb\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
[HKEY_CLASSES_ROOT\.wmv\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"

Note that the @= is just setting the default value for the {NUM} key.