Correct password not accepted

After managing to upgrade from XP home to XP Pro the welcome screen has “click on user name” there is only Administrator there with a password box. I put in my password and it will not accept it. I am so frustrated by this. I reinstalled XP and still came up with the same window. What can I do. Found no help at microsoft to this. Please dont tell me to click the start button because I cant get to the desktop to do this.
Thankyou
clusie