I've known at least 2 other people besides myself where XP just eats itself and refuses to load one day.
For what it's worth - I also had this problem (the system file would somehow go "missing or corrupted") numerous times, like once every other boot up, and so did a lot of other people. I figured a quick and dirty way around it but it was still a pain when it happened. Microsoft's solution at the time would be to tell you how to restore your system without ever addressing the cause - probably because they didn't know. I think it had something to do with the timing between the ram and hardrive during shutdown. Anyway, since I installed the service pack, I haven't had it happen once.
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Of course I could be wrong... but really, what are the chances of that happening twice?