Probably the new Product Key does not match the version of windows XP that is installed on your computer. You have apparently used an OEM key with a non-OEM installation.

The best thing it's to find a product key that fit your version of Windows XP or to re-install with the CD that came with your OEM product key.

You may also try the solution from this thread:
After 'repairing' XP Pro install Activation doesn't run

1. Turn On your PC
2. Press F8 repeatedly and go to Safe Mode
3. Logon as per normal
   (note: Administrator password is usually blank, but hopefully you know your password)
4. Allow the Desktop to fully load (answering "Yes" to continue into Safe Mode)
   (note: This is just a long way of saying go to Safe Mode)
5. Click on Start - Run
6. Type in exactly: rundll32.exe syssetup,SetupOobeBnk (and click OK)
   (note: one space, after "rundll32.exe " also the Oo are two letter o's in "oobe"; capitals and comma are important)
7. Wait a couple of seconds (although you will Not see any change)
8. Restart your PC normally (Start - Shutdown - Restart - Ok)
9. You are now able to go to Normal Mode again (For another 30 days !!!)
10. Do whatever you like, you're back to Normal.

With all the uncertainty and doubts, I decided to conduct an experiment. I installed a clean Windows 8 Pro copy (a copy that I own and with a key that wasn't used anywhere yet) on a computer without Internet access. Here's what I found.

You don't have to activate Windows 8

1. It is true that the installer requires you to enter a valid Windows 8 key before you can continue with installation.
2. However, the key isn't activated at install time and the installation goes just fine without Internet connection (or calling Microsoft).
3. After the installation finishes, you can create your local account (connecting Microsoft Accounts is unavailable because there's no Internet connection) and you land in the Start Screen, which isn't any different from the Start Screen of an activated copy.

Don't expect your life to be easy, though. Microsoft will try to encourage you to activate your copy of Windows 8.

Reminders and Limitations

1. The first reminder you'll get will probably be an Action Center tooltip in the Desktop Mode:

   

   You can safely ignore this tooltip and nothing bad will happen.

2. Another reminder you'll probably see quite often is every time you open the PC Settings screen. When you do, you're greeted with a new section:

   

   Again, you can safely ignore this and click any other settings section on the left-hand side of the screen. Except that...

3. Personalisation settings section is disabled:

   

   The disabled settings include:

   • Your lock screen wallpaper and apps
   • Start Screen colour and background
   • Your account picture

   The message says:

   You need to activate Windows before you can personalise your PC.

   I heard there are some workarounds for this, but as this was not a part of my experiment, I will not describe them here.

4. Every couple of hours, you'll see a full-screen activation request:

   

   To dismiss it, you have to click Go to PC settings and then you're back to the screenshot from point 2. Still, you can exit this screen normally, with Win or Win+D, or any other way you like.

   I wasn't able to pinpoint the exact frequency with which this overlay appears, but it's certainly more than 5 hours and less than 7 hours, so I guess it appears every 6 hours.

5. When you try to enter a command to show the status of activation:

   %windir%\system32\cmd /c slmgr/xpr
   

   You get a not very informing message, saying that Windows is in Notification mode:

   

   EDIT: As vhanla notes in the comments, if you run:

   %windir%\system32\cmd /c slmgr/dli
   

   you'll see:

   Name: Windows(R), Professional edition
   Description: Windows(R) Operating System, RETAIL channel
   Partial Product Key: XXXXX
   License Status: Notification
   Notification Reason: 0xC004F009 (grace time expired)

   But even though the grace time is already expired from the start, Windows continues to work correctly.

So... I guess that's it. There are no other differences from the activated copy of Windows 8 Pro that I found. Of course, because I didn't enable Internet access, online-related activities like installing new apps from Windows Store or obtaining system updates obviously don't work.

TL;DR

• You can install Windows 8 Pro without an Internet connection
• Windows 8 Pro without activation is usable
• There is probably no time limit for activation of Windows 8
• You'll get occasional reminders to activate, but they are safe to ignore
• The only thing that's disabled is Personalisation Settings screen.