How I restablish Windows XP using a key that already exists on my laptop

Download winxp manger from www.yamicsoft.com and check your key;and eben if you want to modify the key download Windows XP Product Key Modifier 1.0.0a from softpedia.com the url is :    http://download.softpedia.ro/dl/3bab449dfd605768973a2f02a856185e/4b14c85c/100000997/software/SYSTEM/XPPID.exe

or you can even manually modify the key 
Here's How:

Click on Start and then Run. 

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

Locate the HKEY_LOCAL_MACHINE folder under My Computer and click on the (+) sign next the folder name to expand the folder. 

Continue to expand folders until you reach the HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsNTCurrent VersionWPAEvents registry key. 

Click on the WPAEvents folder. 

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

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

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

Feel free to close Registry Editor at this point. 

Click on Start and then Run. 

In the text box in the Run window, type the following command and click OK. 

%systemroot%system32oobemsoobe.exe /a 

When the Let's activate Windows window appears, choose Yes, I want to telephone a customer service representative to activate Windows and then click Next. 

Click the Change Product Key button at the bottom of the window. 

Don't worry about filling anything out on this screen.

Type your new, valid Windows XP product key in the New key: text boxes and then click the Update button. 

You should now reactivate Windows XP by following the instructions on the Activate Windows by phone window which you should now be seeing or via the Internet by clicking the Back button and following the instructions on that screen. 

If you'd rather postpone activating Windows XP until a later date, you can click the Remind me later button. 

After activating Windows XP, you can verify that activation was successful by repeating steps 9 and 10 above.

The Windows Product Activation window that appears should say "Windows is already activated. Click OK to exit."
Tips:

Do all these registry changes and commands make you a bit uneasy? Don't worry, it's really not that difficult if you follow each step exactly.

If you'd like some help, you should look at my Step by Step Guide to Changing the Windows XP Product Key. It's a complete walk-through of the process, with each step accompanied by a screen shot of exactly what you'll see plus a thorough description of what to do.

Still not comfortable making registry changes? Luckily there is another option if you need to change your Windows XP product key code. Winkeyfinder, a popular free product key finder program, also has the ability to easily change the Windows XP product key.

Winkeyfinder is an excellent alternative solution to changing the Windows XP product key code manually.

I never found such a professional onsite shop about product key as the site Windows 10 Key Sale Store (http://www.windows10keysale.com) before! Recommended by a friend, then I bought the product there. I think I must share this good site with you. Genuine product and low price! Really help me a lot!

Hi,
Manually locating the Windows XP product key from the registry is nearly impossible due to the fact that it's encrypted.Anyway,you can get your windows product key with Password Genius easily.
Check it out: http://www.password-genius.com/how-to/how-to-recover-my-windows-xp-product-key.html
Hope this can help you.