kupzs,
If you still have your product key from your genuine copy of XP, you can try to use the Product Key Update Tool to add the good key to the pirated XP software and this tool will check the XP software to confirm that no binaries have been modified/tampered. If there have been modifications, the tool will not work and in this case you would need to reinstall the
genuine XP software and the genuine XP key.
You can attempt to change the key using the Product Key Update tool below. The Genuine Advantage Product Key Update Tool is only valid for users attempting to change their current non-genuine Product Key to a genuine COA sticker or genuine Product Key – all without a reinstall! http://www.microsoft.com/genuine/selfhelp/PkuInstructions.aspx?displayLang=en
Note: This is provided no core system files are corrupted. If they are you will need to complete a clean install of your system.
If your original XP was for Japanese language, you cannot use that XP product key on another XP software of a different language because it will not work.
So depending on the status of the machine and whether or not the Product Key update tool works, you might and or might not have to reinstall the original XP software with the original product XP key.
I hope this has helped!
Lori MS