Vista - Laptop freezes on start up

Sometimes (quite often!) when I turn on my laptop it will boot up with the HP
logo, then it won't do anything. It doesn't turn off, it just stays with a
black screen. Sometimes however I can get further then that at the loading
bar comes up with Microsoft Corporation written underneath, the green bit
moves then it goes black and stays that way. Even more rarely it will boot up
to the login screen and when I type in my user password and press enter the
will freeze. Sometimes it won't even load the HP screen it will just do
nothing. I have just taken into a repair shop to have some damage repaires, I
am fed up of taking it there anymore. So can anyone help?? Thanks

(oh and sometimes the power light comes on and it just flashes at me and the
laptop does nothing!!)