My laptop does the same. It must be a hardware fault. I am busy working then WHAM! the computer has instantly powered off. I lose whatever had not been saved. On powering on again Windows (Vista) complains about the computer not being shut down properly. Unfortunately I bought the computer when I was in England so it isn't convenient for me to get it repaired (I live in France).
The fault seems to strike at random. The computer can work fine for a couple of weeks then have the same problem two or three times in one day. There is no logic to it.
Do any of you hardware experts have any idea what the problem is?
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I've left WP indefinitely.