Windows xp is not booting.

windows xp is not booting. the computer is turning on and displays the p5vd2mx welcome screen, but then goes to a black screen with a flashing white line at the top left corner of the screen. furthermore, when i restart and hit the "del" key to enter the setup menu it freezes... the computer has been working fine until now. it has had 2gb ram, 500gb hd and a firewire port put in since it was purchased, however they have all functioned perfectly? what do i do?