Log on as a user with admin privilege.
In control panel, run 'users and passwords'.
Find the appropriate user name and select it. Click the 'set password' button towards the bottom of the dialog box.
In the new password dialog box, enter a new password, twice. Click OK.
Click OK to exit the program.
Your password has been changed.
If you have forgotten the administrator password, and cannot log on as administrator, then you need to reinstall.