In the reviews I've read about desktop search programs, Copernic seems to come out top. I ditched Google Desktop some time ago, since it seemed to consume a lot of system resources. Copernic is certainly as fast, but has the disadvantage that the only mail it can search is if you use Outlook - which I don't. However, Google's own mail search is good enough in these circumstances, although I did find it occasionally useful to have an attachment to an e-mail message pointed out to me by Google Desktop.
I've been trying both Copernic and the relatively little known Filehand and find the latter very interesting. It ouputs the results with extracts to show the context of the search terms and you can vary the amount of context you get. Consequently, it is very easy to tell whether or not the item found is the one you are looking for - not always the case with Copernic. However, Copernic is easier to use, in my experience, if you are looking for, say, an MP3 file rather than all files. I haven't quite figured out how the equivalent in Filehand works, which suggests that it is not very intuitive.
At present, however, the probability is that I shall keep Filehand on the computer - unfortunately, the originators have had to go back to full-time jobs: as they say, they couldn't develop a free desktop search program in opposition to companies with multi-million dollar research budgests. However, they keep an eye on the program and if, one of these days, someone comes up with an offer for them (how about it, Sun or an open source agency?), development will be resumed. For now, however, it is free, and worth considering.