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#1
Check Bios to make sure that USB Legacy is enabled. Try a different USB keyboard. Otherwise you will have to use the Mouse Keyboard on the screen to remove the Keyboard driver and reboot.
You can't just stick a keyboard in while WinPE is running and expect it to recognize it. Your PS/2 keyboard may very well work, but you might want to reboot with the keyboard plugged in, to find out. Then, when you boot into the installer it may very well work. You should also check out this superuser post which provides several useful tips for troubleshooting keyboard and mouse problems while booted into the installer: Windows installation media cant detect keyboard and mouse - Super User.
HTH,
--Ed--