Summarize this article:

This will open the "Run" program from which you can access your PC's registry.  Doing so will launch the Registry Editor tool. You'll need to navigate through several of the folders listed here in order to reach your destination. Since the registry holds most of your computer's sensitive system information, consider clicking File and then Export in the top left corner of your screen to back up your registry before proceeding. Note that you'll need to click the + sign to the left of the folder to expand it--don't click the folder itself. Each folder you need to expand after this point will be inside of the previous folder (for example, "SOFTWARE" was inside of "HKEY_LOCAL_MACHINE" and so on).    Don't expand this one. You should see its contents listed in a window on the right side of the Registry Editor page.   These should be several pairs and sequences of random numbers. This should remove the values listed here. It doesn't matter what you put here, but you'll need to keep the format consistent (for example, if you deleted four characters, you'll need to replace that section with four characters). This will save your changes.  Remember, you can hold ⊞ Win and tap R to do so. Exclude the quotation marks. Entering this command opens the Windows XP activation wizard. For best results, simply copy and paste this text into the Run field.  This option should say "Yes, I want to telephone a customer service representative to activate Windows" and there should be a clickable check box to the left of it. If you see a note that says "Windows XP is already activated", changing your key manually isn't working. Please proceed to the Windows Key Finder method.  This is at the bottom of the "Activate" window. Note that you may have to attempt this step a few times with different keys. If you don't know the version of Windows XP your computer is currently running, consult your computer's manual to verify before you proceed with trying the linked product keys. This will generate a new Windows XP ID for your computer. After this process completes, you'll need to confirm your activation of Windows XP.  This will allow you to activate your version of XP quickly. Note that calling Microsoft using the "telephone a customer representative" option likely won't work since Microsoft discontinued support for Windows XP on April 8th, 2014. After you've completed your Windows XP activation, you should be able to use it without getting locked out of the system.

Summary:
Hold down ⊞ Win and tap R. Type "regedit" into the Run field. Press ↵ Enter. Review the options tree on the left side of your screen. Expand the "HKEY_LOCAL_MACHINE" folder. Expand the "SOFTWARE" folder. Expand the "Microsoft" folder. Expand the "Windows NT" folder. Expand the "CurrentVersion" folder. Click the "WPA Events" folder. Right-click the "OOBETimer" entry. Click Modify. Select the "OOBETimer" contents. Tap Delete. Type in new numbers. Click OK once you're done. Exit the Registry Editor tool. Open the Run tool again. Type "%systemroot%\system32\oobe\msoobe.exe /a" into the Run window. Click OK. Select the telephone option. Click Next. Click Change Product Key. Enter a Windows XP product key. Click Update. Click Back. Check the "Activate Windows over the Internet" option. Follow the on-screen instructions.