Work through these steps which are similar for Win8: Troubleshooting Windows 7 Failure to Boot
After cleaning any infection, you'll need to boot the Win8 installer or Repair CD to run Startup Repair repeatedly to see if it will start. If not try the other steps in tutorial.
It would have been better to shrink XP to install to separate partition from booted Win8 installer. Personally I would not go back to a dead OS now but either try to repair or reinstall Win8 from boot as it should have been installed to begin with.
If you can't boot the required disks to disinfect, repair or reinstall then reset the BIOS to defaults: Clear CMOS - 3 Ways to Clear the CMOS - Reset BIOS.
Try using the one-time BIOS Boot Menu key to trigger the disk or flash stick. You will have to get the disk or stick booted to repair, otherwise we'll need to take other measures.