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After shutting down my PC, I usually unplug the main power cable, but the next time it's plugged my PC won't boot until I unplug and replug it several times (twice or more).

Does this happen to be a power supply issue, or motherboard related, and how do I fix it?

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This is almost always due to bad CMOS settings. The most common cause is tuning memory timing and voltage settings to a particular memory setup and then adding or upgrading memory without returning those settings to their defaults.

It can, rarely, be caused by a bad power supply. Even more rarely, it can be a bad motherboard.

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CMOS settings are't touched. But I've updated BIOS several times, hence I think I should make a downgrade to the first release! – Islam Ibrahim Jan 9 '13 at 8:58

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