→Replace Current Password of your PC without knowing it←

Step-1→ Go To RUN then type LUSRMGR.MSC and press ENTER.
Step-2→ Double click on USERS, Then Right click On the user which you want to change the password.

Step-3→Then click on SET PASSWORD.
[Type the same password in both boxes.]

Note=> There is no need to enter current password....

Now, if u want to remove Your Password Then Go To→CONTROL PANEL
and
open USER ACCOUNTS.... Select the user name and remove your password... 
Enjoy.... ^.^