Delete the content of these folders
C:\Documents and settings\[your name]\Application Data\Microsoft\Messenger
C:\Documents and settings\[your name]\Application Data\Microsoft\Microsoft Live
C:\Documents and Settings\[your name]\Local Settings\Application Data\Microsoft\Messenger
C:\Documents and Settings\[your name]\Local Settings\Application Data\Microsoft\Microsoft Live
And try to execute MSN, if it wouldn't launch, then remove it (from Add/remove programs), and reinstall it again.
C u !
►╠◊◊fĀĥĔďŎὫ◊◊╣ ; Appelez moi Fahd ;-)