The search engine companies say that this kind of tools is becoming necessary to locate important files on hard disk drives that are becoming larger and larger.
Also the need to search through e-mail is becoming bigger. A survey from the Radicati Group discovered that up to 45% of businesses' critical information is stored in e-mails and attachments.
It doesn't yet incorporate web searching, although Yahoo has promised that future versions will allow users to search both web-based and desktop data.Source: BBC
The desktop search tool from Yahoo can be downloaded here