Easily set your Gmail, Hotmail, or Yahoo! Mail as Windows' default email client
There are actually several ways to get Windows to do this, but not many are as simple as GmailDefaultMaker. It's a free program that allows you to choose whether you want mailto:// links to open with Gmail, Hotmail, AOL Mail, or Yahoo! Mail during the installation process.
Once the install has completed, you'll never notice GmailDefaultMaker at work -- there's no taskbar or system tray icon. It's still doing its job, however, but like a lot of handy little programs it just works and stays out of the way.
No, this won't let you email files from your desktop with a right-click. Affixa may be the answer if you're looking for a more full-featured solution.