csrss.exe: This process refers to the client and server run-time subsystem. It is a very important process which should never be forcefully closed by a user as it is critical to system operation. Terminating the process will result in system failure. On Windows start up csrss.exe and winlogon.exe are initiated. If either one of these processes is inaccessible or corrupted, the kernel forces a shutdown process followed by a restart, which then displays a blue screen of death message. The error code which is generally tied to this process being unavailable is 0xc000021a.