If you start using Windows Phone 7, you will need to have and setup a Windows Live ID. The first ID you use will be its default Live ID, and your Phone will be registered against this Windows Live ID.
You will require this ID to access Microsoft services like Marketplace, Xbox LIVE and Zune Pass from your Windows Phone 7.
Q: what if you decide to change your default mail Windows Live ID?
A: Well there is no way to change this Live ID, and the only way to do it is to go for a Reset! After a reset, you can use the other Id you want to. The other additional email accounts can of course be removed without the need to reset the phone.
How To: To Reset your Windows Phone, flick and go the App list page and scroll down to Settings. Open it and tap on About, which may appear towards the end of the list. Here you will see the button: ‘Reset Your Phone‘.
Tap it and your Windows Phone will reset itself back to factory settings!
Important: Resetting your Windows phone back to the original factory settings will erase all personal data, apps photos, videos, music and other content and it may not be possible for you to transfer purchased content to a different Windows Live ID.
You may also want to check out this article Windows Phone 7 – Tips and Tricks, Help and How-to!