Windows Update and Verification

I don't know if this has been talked about before, as it's the first time
I've used this "forum" but if anyone can help, it'll be appreciated.

I'm just curious about something in regards to my "Windows XP - HE", if for
instance, I build a new system, and want to use that as my main one, and
remove XP from my old system, how would I go about "avoiding" the

"Re-Register: can't register this version of windows as it's already been
registered", which I've seen happen, and I jus want to "transfer" that
registration from my old system to the new one, with a clean install.

My copy is legal and I have my own PID from MS, as well as Software Code
from the box. Would I have to unregister something for it to "allow" it on
the new system, or would it be a case of using my PID on that system..

Not sure how XP goes about registering "existing" copies of XP I own, on a
new system, without it demanding the old system I no longer use.