In both Windows NT and Windows 2000, the .jpg thumbnail view is controlled by the following registry Key:

HKEY_CLASSES_ROOT\.jpg\ShellEx\\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1\}

The unnamed Value Name at this key should have a type REG_SZ (String) data value of:

\{7376D660-C583-11d0-A3A5-00C04FD706EC\}

NOTE: As HKEY_CLASSES_ROOT is an alias, you can also use HKEY_LOCAL_MACHINE\Software\Classes.