Update 3/24/2017
Agent Ransack (Windows) is pretty good. Sometimes more useful than windows search. Should test side-by-side.
Update 4/27/2012
You can use windows search to search inside specific file types (e.g., do, ado, doc)
Update 9/16/2011
Nothing is better than Google Desktop. Get it from here if you can't get it from Google.
(3/23/2017 no longer available from oldapps)
Update 9/7/2011
I tried Copernic Desktop Search on my netbook. It took about 3 days to compile a database. During which the machine was not usable (might have been able to speed it up with more judicious use of insomnia). Even after setting it up I thought the computer was running more slowly. I gave up on it and uninstalled it. There are some other alternatives are also possible (see here).
Now I think I'm going to try to blend Launcy and wingrep (see comments)
Effective 9/14/2011 or thereabouts, Google Desktop will be discontinued by Google. So I've already uninstalled Google desktop from all of my computers (using Revo Uninstaller, of course). This is really too bad. I used Google desktop extensively to search for files with specific content. It was really handy with Larry's anytextfile indexer add-on to index syntax files (do's, ado's, etc.) to find and re-use bits of code.