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When turning on your Windows XP the following error could appear preventing the system to boot: “A problem is preventing Windows from accurately checking the license for this computer. Error code: 0×80070002″. This is due to the system drive letter that has changed or to some third-party software that has changed the default security provider. To fix this issue follow one of these two tricks, depending on if the drive letter has changed or the default security provider has been changed.

Reset the Default Security Provider in Windows XP

1. Press the F8 key during startup and start your PC into Safe mode.
2. Start Registry Editor (Regedt32.exe).
3. Delete the following registry keys in the Windows registry:HKEY_USERS\.DEFAULT\Software\Microsoft\Cryptography\Providers
HKEY_USERS\S-1-5-20\Software\Microsoft\Cryptography\Providers
5. Restart the computer.

Match the Default Security Provider to New Drive Letter

1. Run Registry Editor (Regedit.exe).
2. Navigate to the following registry key:HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Defaults\Provider
3. You will see a few Provider key beneath the branch. In each Provider key, edit the Image Path value to match the new drive letter.
4. Exit Registry Editor.
5. Restart computer.

Reset the Drive Letter of the System Drive

Click Start, click on Run… edit: regedit and find and change this registry key to change the drive letter to its original one:

HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices

where \DosDevices\C: and other values presents the drive letters.

Source: tipandtrick.net