What is it?
csrss.exe - Client/Server Runtime Server Subsystem

What does it do?
This is the user-mode portion of the Win32 subsystem (with Win32.sys being the kernel-mode portion). Csrss stands for client/server run-time subsystem and is an essential subsystem that must be running at all times. Csrss is responsible for console windows, creating and/or deleting threads, and some parts of the 16-bit virtual MS-DOS environment.

You will not be able to end this through task manager!

More info

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Virus Precaution:

The csrss.exe which is from Microsoft is located in the c:windowsSystem32 folder. We've been able to find one virus that runs as csrss.exe to fool you.

Nimda.E - Symantec Corporation