I've been a long-time user of X1 Technologies desktop search (more here). There used to be a free version of X1, but now they only sell the "professional" version for $50. I'm telling you right now, it's the best $50 you'll ever spend.
I know Yahoo and Google provide free desktop searches. I've tried them both, and I find the interface on the X1 product much more user-friendly and easy to navigate. (Interestingly, X1 provides the technology for Yahoo's tool.) But if you're not inclined to pay for desktop search and you're not yet using it, get it NOW! Once you've got it, you'll never know how you lived without it.
On a fairly regular basis I find myself searching for things I haven't accessed in years. Just today I had to find an email receipt for a Palm program I purchased more than a year ago (since I just updated my Palm software, which conveniently wipes all programs from your Palm). Using X1, I found the email receipt immediately. I would have never found it without a desktop search tool.
Full disclosure: EDM is an independent third-party consulting firm. This means we have no financial relationship with any of the vendors mentioned in this blog. We provide unbiased opinions on what we see.