A Windows Live ID is also the email address created by user to access Microsoft products and services, so if the user can remember the email address, they will have found their Windows Live ID. If the user can not remember their email address, they should contact Microsoft customer support.
When contacting customer support, the user will be asked a number of questions. Some personal information will be required. Additionally, the user will be asked for the answers to several security questions the user was asked to answer during the initial creation of the Windows Live account. Customer Service may be reach via email or chat.