In Microsoft Windows 2000 and earlier versions of Windows, all files are searched for the content that you specify. Windows XP does not search all file to enhance the performance of searching and to avoid extraneous results. This behavior occurs whether or not you use the Indexing service when you search. Therefore, Windows XP has a known issue for not finding a number of File Types when you do a Search for Files "containing text" or using the "A word or phrase in the file" option.

This can be remedied for many file types, but not all. The cause is the lack of a PersistentHandler value in the Registry for this type of file.
Method #1

Download this VB Script (Written by Doug Knox). Save the file you just downloaded to your hard disk. Locate the file you just saved and double click it. Enter the file extension you wish to modify. If the PersistentHandler sub-key exists, no changes will be made. If it does not exist for the file type in question, the Registry will be updated accordingly. Reboot when you're finished with the file types you wish to change.

(Press Window +F to open the Search dialog box and see for yourself).
Method #2

MS KB 309173 has another fix for this bug:

To configure Windows XP to search all files no matter what the file type, obtain the latest service pack for Windows XP (currently SP1a) and then turn on the Index file types with unknown extensions option.

If you use this method, Windows XP searches all file types for the text that you specify.

This can affect the performance of the search functionality. To do this: 1. Click Start, and then click Search (or point to Search, and then click For Files or Folders).
2. Click Change preferences, and then click With Indexing Service (for faster local searches).
3. Click Change Indexing Service Settings (Advanced). Note that you do not have to turn on the Index service.
4. On the toolbar, click Show/Hide Console Tree.
5. In the left pane, right-click Indexing Service on Local Machine, and then click Properties.
6. On the Generation tab, click to select the Index files with unknown extensions check box, and then click OK.
7. Close the Indexing Service console.

Network administrators can configure this setting for the current user by modifying the registry. To do this, set the FilterFilesWithUnknownExtensions DWORD value to 1 in the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\ControlSet\Control\ContentIndex