Okay, Well this is very easy and does not involve rebooting or logging off, BUT it requires Administrative Access.
#1. Open up Command Prompt.
#2. type in "net user Administrator (Password goes here)" (Without the quotes, And put in a password in place of the "(Password goes here)") So lets say you want to set the password to "computers", You would then type in: net user Administrator computers Simple as that! And doesn't involve rebooting!