windows cannot start missing or corrupt

"Windows XP could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM"
Hi all.
I have had this problem before and each time I follow the instructions of the microsoft website to fix it. My problem, like a boomerang it just keeps coming back! I am using a 200GB maxtor Sata drive on Asus A8V Deluxe Socket-939 with a athlon64 3000+. Firstly why is this happening (as the blaster worm is never found) and secondly how do I stop it
thankyou

I defrag my drive about once a week, so I should hope thats all ok! It turns out tho that I had another virus on the drive which norton did not find out. I decided to run the drive on another pc and do the virus scan and that got it. I hope its dead and that that will solve all my worries! Thanks to anyone who had a good think about it tho good vibes helped me along.