Shutting down winXP

Sometimes when I try and shut down xp my computer locks up and then the only way I can shut it off is by depressing the on/off button.This problem started after I had sent my notebook into HP for repair and when I got it back, after I told them I didn’t want them to fix the missing plastic door on my DVD drive for $350.00, I got the computer back with the hard drive wiped clean except for the OS installed.
Can some one give me somewheree to start?