I should also mention that I have never registered or validated my copy of XP. Never have - never will.
As far as I know, the only time Windows XP doesn't require validation after installation is when it's a hacked illegal version that bypasses this. You're not required to register, but an authentic version of Windows XP requires validation or it will nag you to validate until you do. Plus you can't get any updates or downloads until you do.
If, by some feat, your Windows installation
does work after you swap it over, and if it is genuine, it will state that there has been a significant hardware change and re-require validation.
I know when I upgraded this system (changed motherboard, processor, RAM, sound card, and video card) when I booted it again to reinstall Windows, to my surprise it booted right up into the previous installation, but it still required validation again because it detected the hardware change. It does occasionally work to use an old installation, but this isn't usually the case, and if it does work it will ask to validate.