Dave,
This is a known problem - Q309447. Here is a quote:
Content Search Does Not Search All File Types for the Specified String (Q309447)
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The information in this article applies to:
Microsoft Windows XP Home Edition
Microsoft Windows XP Professional
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SYMPTOMS
When you use the Search All Files and Folders feature to perform a content
search, or you search for a string of text in a file, not all file types are
searched for the specified string.
CAUSE
This change was designed to return common document types such as Microsoft
Office, MIME, HTML, and plain-text files. The search functionality in
earlier versions of Windows searches all files for the specified string and
may return a large number of irrelevant files such as program and
configuration files.
As a result of this change, the search functionality can find the same set
of files if the Content Index service is turned on or off. In previous
versions of Windows, the computer exhibited
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And another one related:
Search Tool Does Not Search for Some Words or Phrases in Text Files (Q318517)
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The information in this article applies to:
Microsoft Windows XP Home Edition
Microsoft Windows XP Professional
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SYMPTOMS
Shell Search may not search for certain words or phrases in text files.
RESOLUTION
A supported fix is now available from Microsoft, but it is only intended to
correct the problem described in this article and should be applied only to
systems experiencing this specific problem. This fix may receive additional
testing at a later time, to further ensure product quality. Therefore, if
you are not severely affected by this problem, Microsoft recommends that you
wait for the next Windows XP service pack that contains this fix.
To resolve this problem immediately, contact Microsoft Product Support
Services to obtain the fix. For a complete list of Microsoft Product Support
Services phone numbers and information about support costs, please go to the
following address on the World Wide Web:
http://support.microsoft.com/directory/overview.asp
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Although the problem is easy to repro on XP and XPe, this seems to be a general XP issue (a bug). You may want to post the question
to microsoft.public.windowsxp.general newsgroup for more info.