When a free build of Windows NT is installed, there is a bit in the registry that if set, prevents debugging the Windows subsystem (csrss.exe). Before you can debug user-mode drivers that are a part of this subsystem, you must clear this bit and then reboot the machine. This bit only needs to be disabled on the target machine. To clear the bit, use the registry editor to edit the following registry key:
HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Control \Session Manager
Edit the GlobalFlag value entry (REG_DWORD) to clear the bit FLG_ENABLE_CSRDEBUG (0x20000).
When a checked build of Windows NT is installed, the CSR-disable bit is cleared, which lets you debug the Windows subsystem.