Ask what the password is...
When you say, "they", are you talking about your parents? They probably installed some parental control or a keylogger or something on your pc.
If your parent's have reason to believe that some of the stuff you are doing on your computer is wrong, then you must be doing some stuff that is wrong and you don't want them to know about it.
My advice: Buy your own computer then you won't have to deal with someone worrying about what you are doing on their computer.