Client Server Runtime Subsystem, or csrss.exe, is a component of the Windows NT family of operating systems that provides the user mode side of the Win32 subsystem and is included in Windows NT 3.1 and later. Because most of the Win32 subsystem operations have been moved to kernel mode drivers in Windows NT 4 and later, CSRSS is mainly responsible for Win32 console handling and GUI shutdown. It is critical to system operation; therefore, terminating this process will result in system failure. Under normal circumstances, CSRSS cannot be terminated with the taskkill command or with Windows Task Manager, although it is possible in Windows Vista if the Task Manager is run in Administrator mode. On Windows 7 and later, Task Manager will inform the user that terminating the process may result in system failure, and prompt if they want to continue. In Windows NT 4.0 however, terminating CSRSS without the Session Manager Subsystem (SMSS) watching will not crash the system. However, in Windows XP, terminating CSRSS without SMSS watching will crash the system due to the critical bit being set in RAM for csrss.exe.
The Windows NT 3.x series of releases had placed the Graphics Device Interface component in CSRSS, but this was moved into kernel mode with Windows NT 4.0 to improve graphics performance. The Windows startup process from Vista onward has changed significantly. Two instances of csrss.exe are running in Windows 7 and Windows Vista.