Some USB 3 drives can cause this issue if you use them to install Windows or the Apple Support Software (drivers). Use a USB 2 drive instead to work around this issue.
MacBook Air (13-inch, Mid 2013) and iMac (Late 2013): Keyboard and Trackpad may stop functioning during Windows 8 install
The keyboard and trackpad on MacBook Air (13-inch, Mid 2013) and iMac (Late 2013) computers may stop functioning at the Personalize step when installing Windows 8.
This document will be updated as more information becomes available.