When you search for files that contain text by using the A word or phrase in the file search criterion, the search results may not contain files that contain the text that you specified. For example, .log, .dll, .js, .asp, .xml, .xsl, .hta, .css, .wsh, .cpp, .c, or .h files, or files with no file name extension, may not appear in the search results even if the files contain the text that you specified. This problem may occur even if you specified the file name or type in the All or part of the file name box.
(Microsoft Support: Using the "A word or phrase in the file" search criterion may not work)
Google for a registry-file named “FilterFilesWithUnknownExtensionsForXP.reg”.