How can I get Windows 8 to stop asking me about mailto links?

Maybe this link will help you: http://support.google.com/mail/bin/answer.py?hl=en&answer=10966

Making Gmail your default mail application

You can designate Gmail as your default mailto handler. This means that when you click on any hyperlinked email address, a Gmail compose window will open so you can send a message to that address from Gmail.

If you use Windows: Download the Gmail Notifier. Right-click the Notifier icon in your system tray, and select Options. Check the box next to Use Gmail for internet mailto: links. Click OK. If you decide later that you don't want to use Gmail as your default email application, simply uncheck the box.