I am posting this as this is what I have observed. Many people still think that for using Windows Live ID services and to sign in to any Windows Live ID site, one has to have mail address from @hotmail.com or @live.com. This is not correct as one can use any existing e-mail address from any e-mail provider but you first have to set it as Windows Live ID for such e-mail addresses. That is first you have to create your sign in credentials. So let us see how its done, For example say you want to use your Gmail as Windows Live ID and you want it to use for Windows Live Messenger.
Go to https://accountservices.passport.net/ppnetworkhome.srf?lc=1033 or just http://www.passport.net/ which will take you to the link. Scroll down to “Sign Up Today” > “Use an e-mail address you already have” > And click on the link Get started now. This link will open up a Create Credentials page. Just fill up with your Gmail address you want to use and set password. This password may not be same what you had given for Gmail address while registering it. Fill the form completely and Press Continue. So this creates your credentials for Gmail.
After that you will get two mails sent to your Gmail address. One will to “Confirm your e-mail address for Windows Live ID” and the other mail “Welcome to Windows Live ID”. So once you confirm your e-mail, you are ready to use your Gmail as Windows Live ID.
Here you can see I am signed into Windows Live Messenger with my Gmail as Windows Live ID.