Sounds like a bad LCD or bad LVDS cable. Cable usually gives itself away by causing an intermittent problem, or a problem that comes and goes if the screen is tilted. A bad LCD usually just stays once the problem happens.
Thanks. The LCD is good and video card is good. For a few times I can have normal pictures. I think that BIOS may have problems but I can not open BIOS page. The screen becomes blue during start. It also show ripples on log-in screen. I hope HP a techniciancan tell me where the problems come from.