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I had the same issue this afternoon. I have used Avast in the past (windows xp, 7) so I thought I would install it on my new Dell laptop (windows 8.1).
I bought the updated internet security and upon installation and first scan all seemed well; however, upon reboot, both keyboard and trackpad became inoperable.
I was close to freaking out until I tried a workaround (I plugged in a usb mouse by the manufacturer of my laptop (Dell) and it was recognized and instantly installed. I used the mouse functionality to uninstall Avast, rebooted my dell and through windows update discovered that for some reason Avast Internet security had removed a registry key necessary to operate my laptop keyboard (something #19?).
I am not going to use Avast if this is the state of their working relationship with Windows.
Hope you had some luck un-bricking your compy.
John F. Adams