Many years ago, in the heyday of DOS, Lotus brought out a desktop search program called Magellan that did several amazing things. It indexed your entire hard drive and kept that index up-to-date. It let you search for anything in any file on the drive, no matter what the format. And it displayed the contents of what it found in a handy preview pane located right next to the list of relevant files.

Magellan's users tended to become fanatical about the program, because it suddenly made the content on the drive more important than the file names. And then, to the gnashing of teeth throughout the Land of DOS, Lotus killed it.

Over the years, a smattering of inexpensive desktop-search programs for Windows have cropped up - the one I used the longest was Enfish Find - but now, thanks to the immense popularity of internet search, users are finally beginning to grasp just how useful searching one's own hard drive can be. Today, desktop search is everywhere - and usually free, as it is in downloadable apps from Copernic, Google, MSN and Yahoo. It's also integrated into Mac OS X Tiger and the forthcoming Windows Vista.

What's the big deal? Same as on the web: desktop search unlocks highly useful information that was heretofore hidden inside otherwise impenetrable files. But how that info is presented and how you can filter that presentation are the keys to usefulness, and that's why I currently use - make that practically live in - Yahoo Desktop Search, the free little brother of X1's US$75 (£44) X1 Desktop Edition.

When I want to find information that somebody sent me about podcasting, I just type the word into Yahoo Desktop Search's search window and instantly see a display of every file on my drive that contains "podcasting". Unlike some of its free rivals, YDS presents those results as a file list with a preview pane.

I can further restrict the search with Boolean operators (AND, NOT, OR), limit the search by type of data (email, attachment, document, picture, instant message), narrow the results by filename or mail sender, and sort by date or other criterion. When I find the nugget I want, I can open it with a single click. All of a sudden, desktop search becomes the place where I go to start looking for anything.

Search proponents have been known to take this power a bit too far: why bother keeping an address book of your correspondents? A couple of keystrokes, and you can extract their phone numbers from their email. Why bother categorising mail into folders when you can search your entire inbox in seconds?

Not so fast. One reason to keep an address book is so that you can transfer it to and from a phone. And one reason to categorise mail into folders is so that you will have everything related to, say, a particular upcoming conference together in a single place. (Not all your mail regarding the conference will explicitly mention it by name; as a result, even a perfect search might miss relevant items.)

But it's easy to become a fanatic about this stuff. Having saved all my email (well, not the spam), I have an amazing database that lets me see what I was doing as far back as the early eighties. Now it's accessible, thanks to a search engine that can look into, index and display files in formats I can barely remember.

I can't stress enough how useful a desktop search program is. Don't wait for Windows Vista. Start with Yahoo's program, and if you're unhappy with it, move along to one of the others. Search is the most valuable software category since the long-gone Age of Magellan.