When you attempt to check the validity of your Windows XP license using WGA (Windows Genuine Advantage), it is not successful, and your receive:
Product Key Inaccessible
This behavior will occur when you do not have the required permissions on the
%systemdrive%\Documents and Settings\All Users\Application Data\Windows Genuine Advantage\Data folder.
To workaround this issue, grant the local users group both Read and Write to the \Windows Genuine Advantage\Data folder:
01. Start / Run / %systemdrive%\Documents and Settings\All Users\Application Data\Windows Genuine Advantage\Data / OK.
02. Right-click the Data folder and press Properties.
03. Select the Security tab.
04. Select Users (<ComputerName>\Users) and check that the Allow column has Read & Execute, Read, and Write checked, and that the Deny column has no checks.
05. Press OK.
06. Try using WGA. If this step is successful, exit this procedure.
07. Shutdown and restart your computer.
08. Press F8 on the Boot menu.
09. Use the arrow keys (with Num Lock off) to select Safe Mode and press Enter.
10. If you have more that one operating system installed, use the arrow keys to select the appropriate installation and Press Enter.
11. Open a CMD.EXE window.
12. Type RD /S /Q "%ALLUSERSPROFILE%\Application Data\Windows Genuine Advantage" and press Enter.
13. Restart your computer normally.
14. Use WGA to validate your license.