If you use a PS/2 keyboard or a laptop (which usually have PS/2 interface) the Setup Assistant will ask you to connect an USB keyboard, just plug one to pass this check, then you can unplug it and keep using the PS/2 one. If you don't have an USB keyboard around you can edit /System/Library/CoreServices/Setup Assistant.app/Contents/Info.plist and remove the lines

Code:

<string>DeviceSection</string>
<string>KeyboardTypeSection</string>

Boot using -s (single user) and run

Code:

mount -uw /
vi /System/Library/CoreServices/Setup Assistant.app/Contents/Info.plist

Position the cursor on the line to be removed and type dd. Repeat for the other line. Type :wq [enter] and reboot.