If your Serial Key or Product Key of Windows XP is blacklisted by Microsoft.com & you are unable to update some major updates, Then Sure you have to purchase XP  New Copy from Microsoft. After Purchasing the New XP Copy you will get a Legal Serial Key or Product Key from Microsoft. If you want to Use that Legal Serial Key or Product Key without Reinstalling your present Windows XP, then here is a Nice Tutorial to change the XP Serial key without reinstalling:

1.) Click on Start menu & Press on “Run”.

2.) In the text box in the Run window, type regedit and click OK. This will open the Registry Editor program.

3.) Locate the HKEY_LOCAL_MACHINE > Software > Microsoft > WindowsNT > Current Version > WPAEvents.

4.) In the results that appear in the window on the right, locate OOBETimer.

5.) Right-click on the OOBETimer entry and choose Modify from the resulting menu.

6.) Change at least one digit in the Value data text box and click OK. This will deactivate Windows XP.

7.) Close Registry Editor.

8.) Again Click on Start menu and then Run.

9.) Type the following command and click OK.

%systemroot%\system32\oobe\msoobe.exe /a

10.) When the Windows Product Activation window appears, choose “Yes, I want to telephone a customer service representative to activate Windows” and then click Next.

11.) Click Change Product Key, Type your new, valid Windows XP product key in the New key text boxes and then click Update.

12.) If you’re taken back to the previous screen and prompted, choose Remind me later.

13.) Restart your PC

Also take a look @ How to Change Vista Serial key without Re-Installing