Bug: The 3D Analyst Tools returns an error, "000816: The tool is not valid" with the 64-bit Background Geoprocessing patch on a Windows XP 64-bit machine
The 3D Analyst tools return an error, "000816: The tool is not valid." This occurs only on the winxp 64-bit operating system and if the 64-bit background geoprocessing patch is installed and background processing is enabled.
This is a known issue.
Run the 3D Analyst tools in the foreground.