FileSearchy is a free (for personal use) desktop search tool for Windows.
The program runs very fast searches for file names. We launched FileSearchy, typed HELP and pressed enter, and it found and displayed details on more than 2,100 files (where the name contained the text HELP) in less than a second.
You can further customise your search by file location (look in just a particular folder or tree), modification date or size range.
Unusually for this type of program, FileSearchy is also able to search the contents of some file types (plain text, PDF, Office). This worked well for us, although if you have problems the authors recommend installing the Microsoft Office 2010 Filter Pack.
If you spot a file which looks interesting, then clicking it displays any text matches and the sentence where they were used (useful for context). You're also able to browse the contents of the whole file, although only in a plain text view (you won't see the images or formatting in a PDF file, say). But this doesn't really matter, as double-clicking any matching file launches it in your default viewer.
There are some unexpected extras. FileSearchy supports regular expressions for search terms. A tabbed interface means you're able to have several searches open at once. And you can copy your search results (or one or more of the files) to the clipboard for easy reuse elsewhere.
A commercial FileSearchy Pro version extends the program further with support for searching the content of eBooks (EPUB, FB2, MOBI), as well as filtering by file category and exporting search results as a CSV file. It's yours for $24.95.