Question about Outlook Express

Someone else is using my email address and i am worried that he may have access to the account i logged on to my drop box using my Outlook email address(my real account uses g-mail but forgot this and logged on with outlook) i done a forgot password request and was shocked to see there was an account made on my email address by someone else who also has my name. I am wondering as to whether he had to do a email verification and thus had access to my account. Could someone help me in keeping this account from being used for sign ups on various sites.(It already has been I had to tell facebook it's actually my email address and close the account.)

P.s its got the point where its really starting to P*** me off its happened for over 2 years now.