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. Open Computer Management (Right-click "My Computer" and select "Manage")
2. Expand Services and Applications and right-click Indexing Service and choose "Properties". (Indexing Service don't have to run for this setting to work)
3. Select the Generation-tab and check Index files with unknown extensions and press Ok
4. 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 KB309173
   

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