If you're the account holder, you can call Blizzard and they will reset your password. Or you can use the forgot password link on the login page for your account settings. that will at least get control of the account back. From there, try a free antivirus/antispyware scanner like
AVG. A lot of trojans wont be picked up by an antivirus, but spyware/malware scanners can pick em up. Thats why its good to use a scanner that will check for both.
For really persistant ones, you may need to make sure its not launching itself at startup. You can see whats launching at startup by opening a RUN command from the start menu and putting in MSCONFIG. That will show you everything thats launching. Some of the really nasty ones will have a service that launches a randomly named executable, which will relaunch the service if its not running. Fun stuff, eh?