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Windows XP changes the filesearch functionality to only to look for registered files. This is done to make the search quicker and more userfriendly. But it can be annoying for users who are expecting that all files are searched.

To make it search all files do the following:

1. Press the Start-button and select Search.
2. Click Change preferences and then click With Indexing Service (for faster local searches).
3. Click Change Indexing Service Settings (Advanced) (The Indexing Service isn't required for this setting to work).
4. On the toolbar click Show/Hide Console Tree
5. In the left pane right-click Indexing Service on Local Machine and select Properties
6. Select the Generation-tab and check Index files with unknown extensions and press Ok
7. The above steps should be reflected in this DWORD registry key:

   [HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Control \ContentIndex]
   FilterFilesWithUnknownExtensions = 1 (Default = 0)
   
   More Info MS KB Q309173
   

To make it use the classic search without assistant, set this STRING value in the registry (Will make it stop accessing the Internet for search tips):

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \CabinetState]
Use Search Asst = "no"

Credits ntfaq.com