FYI, this is exactly what Coveo Desktop search does. CDS (OEM’d from Copernic) is top rated over Google Desktop Search and all the others. Via a small footprint client or toolbar, CDS searches the following—first 8 are on the local desktop:
Email—Exchange Outlook and Outlook Express, Lotus Mail, Mozilla Thunderbird
Files
Audio
Images
Video
Contacts
Favorites—Internet Explorer, Mozilla, Mozilla Firefox, Netscape
History—Internet Explorer, Mozilla, Mozilla Firefox, Netscape
Intranet search via Coveo Enterprise Search including secure search for SharePoint SPS/WSS or any other file/nework/share, Novell Netware, web, or Exchange public folder servers (Lotus Notes soon)—all out of the box; and relevance is *outstanding*
Internet search via AllTheWeb (Yahoo!)—you could make it Google if you really had to
Both CDS and CES have an API to add custom document types as well.
Both are free for evaluation; CES free for up to 5000 documents. See coveo.com for more details.