I am planning to build a site with openID authentication , most of my target users will register in the site using Microsoft accounts (hotmail/live/windowslive).
Is there anyway to use Microsoft accounts as openID?
As far as i know they still only use their "Passport"/"Windows Live ID" system, so no. They have announced however that they are going to support OpenID at one point or another. Take a look at the link below there is even a Tech Preview to test out their implementation maybe that can be used somehow: