Windows Vista Update

after installing this update ''vista service pack 2'' and restart my computer i can't load to my desktop

by private
(italy)

when i arrive at the moment of typing the password..i type it and then it loads..but after it loads it becomes a blank screen and comes back to the moment where i should type my password..i cant load to my desktop anymore..im only using the same mode with the internet connection at the moment..pls help me..