Messenger keeps saying i’m signed on in multiple places which I know I’m not, and I know someone hacked into my account, how do I get them permanently out?

I also keep changing my password but within a day, they get into my account again. I don't know what to do. My operating system was also genuine microsoft 7 ultimate and they have stolen that to where it says my operating system is no longer genuine. I don't know how to kick them out and keep them out because like i said, i keep changing my password and that's doing nothing.