Actually it's worryingly easy to crack a windows password!
But that aside, change your password (as VHG says) and also right click on my computer, select manage, then, in the left pane in the window that comes up, click the little + next to local users and groups, click on the users folder and then in the right pane go through all users listed there and either disable ones that aren't being used or change/set the password on all of them.
You could set the bios password, but be aware that in doing that if you forget that password on some systems it can render your motherboard unusable and your left with no choice to replace it! (or get down and dirty swapping bios chips on the board!)