It would have been helpful to know what email client you use since it's a setting in there you must change - your version of Windows is irrelevant as far as that's concerned.

However, let's assume you are using Windowqs Mail. Go into your email account settings, highlight your Hotmail account and click "Set as default". I can't be more specific as I only have one email account & I use Outlook 2007, but I can see there is a "Set as default" button in my mail account settings (only it's greyed out in my case of course).