Check your logs in Event Viewer for log-in and log-out times. That will tell you if someone is logging on your computer when you aren't there. (That is, if you are logging out/powering down when you aren't at your desk.)
Or, you may just want to change your password by pressing Ctrl+Alt+Delete, and then click Change a password. You'll need to type your old password, type your new password, type your new password again to confirm it.