if you have windows xp, and sp2 go to Start> Set programs access and defaults, and look around there.
or when you download a new browser, such as firefox there will be something in options that allows you to choose it to be the default browser.
If you download stuffplug ng, at www.stuffplug.com you can tell msn to open hotmail in the default browser.
I hope i could help you...even i confuse myself