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

Virus Precaution:

The csrss.exe which is from Microsoft is located in the %Windows/System32/csrss.exe folder.
There have been quite a few different viruses that masquerade as this file. If you find it in a location like C:\DOCUME~1\Home\LOCALS~1\Temp\csrss.exe you will probably need to run combofix to clear this.

Nimda.E - Symantec Corporation