If you find that an unauthorized person has used your Windows Live ID to sign into your Windows Live Hotmail acccount, or any other Windows Live service, then this write-up may help you.
Ask yourself: Did you share it with anyone, inadvertantly? Or did you provide your username and password anywhere? Maybe in some website? If you remember the site or link where this had occurred, please report it as a phishing site and let Microsoft know.
Now if you still have access to your account, then change your password immediately. Also change your Secret Answer & alternate email id if any.
But if you no longer have access to your account, try the suggestions found on the Password Reset Basics page.
You can also visit the Windows Live Validation Page that Microsoft has set up to attempt to validate ownership of an account in cases where the Password Reset Basic steps do not work.
The Windows Live ID Validation Page will ask you key questions about your account. Hence, the more information you had provided while signing up, the easier it becomes to validate.