Can you still access the account? If so, change the password. Do you know the answers to the Secret Questions configured for the account and/or did you configure an alternate contact email address for the purpose of requesting a password reset? If so, go to http://account.live.com and perform a password reset.
If you cannot access the account, you will need to submit a form to Windows Live ID support by going to http://support.live.com
They will contact you via email and will require a significant amount of data to prove that the account was indeed yours. The responding email will explain what you will need to provide.
-steve