Where do you find this file?
Note: The csrss.exe file is located in the folder C:\Windows\System32. In other cases, csrss.exe is a virus, spyware, trojan or worm!
Have a look at this!
http://www.neuber.com/taskmanager/process/csrss.exe.html
http://www.processlibrary.com/directory/files/csrss/
If you feel that is an virus, download removal tool from here & do a scan!
http://www.symantec.com/security_response/writeup.jsp?docid=2001-103004-2955-99
To keep your system secure all the time,
Download & Install the following 3 freewares & Keep them always updated. It forms a free complete security suite for your system!
1. Avira Anvivir -
http://www.free-av.com/en/download/index.html .
Avira is best & light solution for excellent overall system protection & internet security with real time updates, .
2. Malwarebytes - http://www.malwarebytes.org/ .
Malwarebytes is one of the best & effective antimalware tool out there.
3. Ccleaner -
http://www.piriform.com/ccleaner/download .
CCleaner is a system optimization, privacy and cleaning tool with an efficient & most useful registry cleaner.
The Best Security & System Optimization combo for all types of systems.
Always use ccleaner & Disk cleanup for system cleaning, don't do it manually unless you are sure.