I think someone has hacked into my account and is sending regular e-mails from it. How do i stop it?

[Bumbleshank]

As already stated, change your password on that account. The person who's hacked it, likely has a list of account names and matching passwords... changing your password will stop access. Check your security questions and if possible, change them. Since you don't want the person in question resetting the password after you.

The big question is how did this happen? Yes it's possible that they simply cracked the email site itself, but more than likely, you let a script/program run on your computer that you shouldn't. I would do a virus/spyware scan on your local machine, look for any new Addons/toolbars in your browser, and check that you've not agreed to let any silly apps access your facebook data.