Windows – Cross activation between Windows editions (Pro, Enterprise)

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.

Thankfully, I noticed something about all of my Virtual Machines that I hadn't prepared as those guides had outlined.

I ran VBoxManage showvminfo <uuid|name> and noticed that the UUID valued matched the Hardware UUID value, leading me to surmise that if any VM Definition didn't have a uuid attribute in the /VirtualBox/Machine/Hardware node, then the VM would use the "Machine UUID" value as the "Hardware UUID" value. So, for my template VM, it did have a "Hardware UUID" that I could use; I just had to find how to transfer it to any clones I would make.

To ensure that the UUID transfers, you can take one of two approaches:

• You can edit the template VM Definition file directly. The "Hardware UUID" will only transfer to clones if the /VirtualBox/Machine/Hardware has the uuid attribute. So, copy the /VirtualBox/Machine uuid attribute and add it to the /VirtualBox/Machine/Hardware uuid attribute. You probably want to re-initialize the VirtualBox machine database after this, by closing/killing any VirtualBox process you're running. (VBoxSVC sometimes lingers on Windows; you may have to kill it from the Task Manager.)
• You can clone your template and make a new template VM, and then use VBoxManage modifyvm <uuid|name> --hardwareuuid <uuid> to set the new template's "Hardware UUID" to the original template's "Machine UUID"/"Hardware UUID". I say you have to clone the VM to create a new template VM because if you try to set the "Hardware UUID" with this command on the original template, it won't add the uuid attribute to the /VirtualBox/Machine/Hardware node, since it can still imply it from the "Machine UUID", and thus won't copy this value to any clone it makes.