Copernic Desktop Search
Earlier, I compared Google and Yahoo Desktop. At that time, I thought Yahoo was better for the types of searches that I do. I mainly search for text inside 391 (and counting) papers stored on my computer in pdf form.
Now I have found a desktop search that I like even more: Copernic. It uses fewer system resources than Yahoo and looks a bit cleaner.
The first thing that I do when I setup a desktop search is to ensure that it searches only in the directories where I keep my papers. That speeds up the search by making the results more relevant and uses less RAM and CPU in the background.