If you open your Windows Task Manager, you can usually see a process called csrss.exe running. If you choose to show processes from all users, you may see more instances of it. But what is this file doing and why has it caused concern to so many users in recent years?
Most importantly, csrss.exe itself is NOT a harmful process, but indeed an essential background process of your Windows system. It is an important part of the of the Win32 subsystem, and is closely related to a file called Win32.sys. The name csrss.exe is the abbreviation of the "client/server runtime subsystem", and therefore it must not be disabled. This is the reason why you can't find this process in the Startup settings, as its running is critical.
The client/server runtime subsystem is a component of NT based Windows versions, pretty much all versions we tend to use today. So why has this essential background process caused such a panic amongst Windows users around the world? It all started by hoax emails and other forms of online messages claiming that csrss.exe is a harmful process and must be terminated if found.
Well, the intention of the author of these emails might have been good, but was based on a misunderstanding. Claiming that csrss.exe is a harmful process is not entirely true. There are (or mainly were) some viruses, spyware, and trojans which were known to disguise themselves as csrss.exe, including the Nimda.E, the W32/Netsky.ab@MM and W32/VBMania@MM viruses. In these cases, the process that looked like csrss.exe was actually an instance of the virus, and of course could be harmful as such.
However, most antivirus systems can easily detect and quarantine these files today, most probably before you even notice them or let them run in the first place. Long story short, if you find csrss.exe running in your Task manager, do not attempt to stop it, or else your system will crash. However, there might be some other processes on your computer slowing down your system, so you are advised to check your system with a tool that detects unnecessary processes running in the background.