Acer aspire 4720

Question

i bought acer aspire 4720 running on vista ultimate in August 2007. then after 2 months blue screen appeared twice and couldn't enter OS. My vendor changed to window XP. It runs ok until early this month when the blue screen appear again. after third time, it wont enter the OS. it stop at loading. when i use the window xp Cd. it says no hard disk drive detected. i run bios and everything seem ok.