Windows Explorer is a process used by Window 8/7/Vista/XP/older versions to allow a user interface on your computer which means that Explorer.exe is a very crucial service that lets you actually open the drives and folders like My Computer or My Documents. This process MUST run on your system unless you prefer using it via command line only. It comes in with any Microsoft Windows operating system and is launched at every windows start up. Nope, you don't need to open it manually, it gets launched by itself when you login to your computer. Located in C:\Windows, this process helps you a lot with your graphical user interface (GUI). It decides the way your system needs to look. Let's look at all those that come under this process.

1. Taskbar: Yes, the Taskbar works only if you have this process running. This includes the start menu, quick launch icons and all that you have stuck to that pane!
2. This is also nothing but your File Explorer in Windows 7 and Windows 8. The process lets you navigate through the files and folders present in your hard drives which makes it the most important feature of Windows giving it the GUI advantage over Linux/Unix based operating systems.
3. Many people are not aware that this process is also used by other external non-windows processes to provide the services assigned to them.
4. Folder options: This is easy to understand since everybody already know how to hide and unhide files and folders using these options. These options not only hide your secret files but also provide a lot of other services too.
5. Did you ever wonder what that "Map Network Drive" really meant? Let's say you have a storage unit or a server attached to your PC. This option lets you have a piece of that Server's Hard drive and guess what, this also needs your Explorer.exe process.
6. You can change the icons and make them the way you want them to look. Right click on your folder, click properties and change the icons to have your girlfriend's pic on them.
7. Not only that, this process lets you arrange and sort the icons in a long page letting you manage the files and folders inside a folder.

Those were what Explorer.exe allows you to do. There are more of what all applications use this process to execute. These include VIRUSES! Yes, that's right! If you are new to dealing with viruses on your own, you would not even have a clue that these are actually getting injected into your Explorer.exe process. In these cases, your Anti-viruses don't work. Why? That's because this process is always running on your PC that whenever an Anti-virus tries to delete this code, it keeps getting denied access since the process is already in use! I'll tell you how this is done. When a virus executes in your PC, it runs a script adding itself to your Explorer.exe, corrupting your files and hiding folders. This can be done without injecting the code into the process but unfortunately, the creator of that virus has his intentions. This is quite common now and situations like Explorer.exe being unavailable are also seen in scenario where these viruses delete Explorer.exe from your Windows folders. This can be done by using a process editor and erasing the virus details from the exe file but I would say its totally not advisable. Like I said, this is a very crucial process. So how about you Google it and freely download it from the Internet? I am sure that sounds simple! If you are doing so, the file is present in C:\Windows and that's on Vista, Windows 7 and 8. You need to replace the C:\Windows\explorer.exe file with the new Explorer.exe that you download. Once you are done updating it, you can use your Antivirus to delete the viruses that were troubling you all along.