One cannot "register" a pirated copy of Windows XP.
Only way to fix the problem is to perform a repair install
using a new, legitimate, "full version" Windows XP Pro CD
and Product Key. Afterward, visit the Windows Update web
site to download all the critical patches and updates.
Don;
You need to buy a legal copy.
Depending on the version you buy and the version installed will
determine the procedure used to install the legal version..
You need to purchase and use a legitimate full retail copy of
WinXP Pro to perform a repair (a.k.a. in-place upgrade) installation,
using the new CD and Product Key.