Bootcamp upgrade Win 7 Pro 64 to Win 8 fails

I'm trying to upgrade Win 7 to Win 8. Install goes to 90% then message -"your computer will reboot in a few moments" then a few minutes later message

"install failed" and it rolls back to Win7.

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This is on an early 2012 17" Macbook Pro I7 2.4 running Win 7 Pro 64. I installed the new bootcamp drivers, tried disabling all unnecessary devices-audio/lan/bluetooth etc,, tried clean boot, disabled video card all with the same result.

I ran the System Update Readiness Tool and retried the install with the same result.
Tried install from desktop and DVD
Hope someone can help-thanks