When I run setup /prepareSchema in my Domain (2003-native, WIndows 2003 Server SP1 german) setup fails to access the regKey at HKLM\System\CurrentControlSet\Services\NTDS\Parameters with "The requested Registry access is invalid" (translated from german) and stops. I can access the key via RegEdit. Any Ideas?

As I found out, I could not access the HKLM *remote*! Although setup was running on the schema master it obviously tried to remote connect to it. KB 555335 helped me with correcting the security settings on registry in the "Default Domain Controllers Policy". Now /prepareSchema is running!