Hardware malfunction. my laptop dell Inspiron 6400 is experiencing problems right now. It stop and

Question

my laptop dell Inspiron 6400 is experiencing problems right now. It stop and hang and a big blue screen is showing these message:

HARDWARE MALFUNCTION
CALL YOUR HARDWARE VENDOR FOR SUPPORT
NMI : PARITY CHECK / MEMORY PARITY ERROR
THE SYSTEM HAS HALTED

Computer is a Sony VGN-N325E running Windows Vista XP Home. I just use it for e-mail and downloading and copying pic from my Digital Camera. I've gotten this message at least four times. I restart the computer and it starts up fine and then suddenly it happens again. What is going on?