It can be found here:
HKLM\Software\Microsoft\Cryptography\MachineGuid
You need admin permissions to change it.
Here's some C# code to do it:
var machineGuid = Registry.LocalMachine; machineGuid = machineGuid.OpenSubKey(@"SOFTWARE\Microsoft\Cryptography", RegistryKeyPermissionCheck.ReadWriteSubTree, RegistryRights.FullControl);
No idea if it will change the windows auth key.