If you have the restore CDs then the documentation that came with the desktop should show you how to do it.
I think if you turn the PC on, put the first restore CD in the drive, then turn the PC OFF (leaving the CD in the drive).
Then when you switch the PC back on it SHOULD boot from the CD and the restore should continue.
If this does not work you may need to set the CD drive to boot BEFORE the hard drive. You do this in the BIOS.
Your PC documentation should show you how to go in the BIOS.