Recovering from a failed restore with Q

Hi,
I have a customers pc in my shop. In the past I had a Q pc that I tried to restore and it was interupted in the process. I cant remember where I got this info but I know you can hit certain keys while it boots and it will begin again. Right nowwhen I insert the cd and reboot the screen is black with a cursor flashing and it wont get any further than that.Could you tell me which keys to enter for it to work.
Thanks