Windows XP file search has been "improved" so that it no longer works for finding text in file types other than standard document and media types. Refer to the following knowledge base article 309173:
http://support.microsoft.com/kb/309173/EN-US/
Here is an excerpt: "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."
If you are a programmer or power user, these "extraneous results" are likely to be the files you want. I have found that searching for text in certain file types still fails for some file types even after making the suggested configuration change. I don't know why Microsoft doesn't provide an option to fall back to the old-style brute force search (that is, searching all matching files for the text entered, which is kind of what you would expect it to do in any case). If anyone knows how to do this, please chime in. Meanwhile, I don't rely on Windows' text search anymore. It's worse than useless.