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I do this once a month on the PC, I admit I've not done it for ages silly me, if you're PC crashes and it does'nt run right on boot-up, go back to a restore point you've created and it should fix the problem.
XP does it automatically (when you add software etc. I assume)
Thank goodness!
eta - occasionally I`ve had to turn off sytem restore to get rid of a trojan though. But it has saved my bacon a few times.