The problem for you (and for me) stems from the old XP hard drive not being able to boot in the new hardware. You need to either start over with a new full version of Windows 8 or you need to "transplant" your XP disk contents to the new hardware while still allowing it to work just long enough to start the Windows 8 installer.
I have two valid full-version windows installations on old hardware that I would like to port over to my new hardware. One is Windows XP and the other is Windows 7. In both cases, I do not need to keep the original OS, apps, or data. They're clean licenses that I would like to not have go to waste.
I kept the old hard drives and system components and have thought of a way to do it with two Windows 8 upgrade discs. Untested. But might be tested sometime in the coming months. You get what you pay for...but here's my thinking:
R1. Old system must still be bootable.
R2. You must have Macrium Reflect with the bare-metal restore feature.
R3. New hardware must be built and ready-to-go.
Steps I plan to take:
1. Boot original system. Install Macrium Reflect and create a boot disk/USB stick that contains the Windows PE environment.
2. Take a backup image. Should be able to do it with Macrium inside of Windows, but just to be safe, I would take two backups; one "cold" with a boot disk and one "warm" from Windows.
3. On the new system, restore it with Reflect's "redeploy to new hardware" option.
4. Boot it on new system. Validate that XP still works and is properly licensed. Correct any outstanding issues.
5. Now "upgrade" it to Windows 8 with your upgrade disc and key.
6. De-install/Dispose of the original XP and/or repurpose the original XP hardware.
No guarantees of course!