Box.com is offering 25GB of free storage to new cloud storage users. New users can sign up via the web to get their free storage. You will have to make a new account. You existing account can't be upgraded.
Box has done this before via the Box app for iOS, Android and BlackBerry. I managed to snag 50GB of free space a few months ago.
Box has recently released a Windows 8 app. Don't forget to install it from Windows Store after signing up.