A system file is any file with the system attribute turned on.
A file or folder with the system attribute toggled on implies that Windows or some other program sees the item as being crucial to the overall function of the operating system.
Files and folders that have the system attribute logged on should usually be left alone. Changing, deleting, or moving them could cause instability or complete system failure. For this reason, system files typically have the read-only attribute, as well as the hidden attribute, flipped on as well.
The most popular system files you might have heard of on a Windows computer include kernel32.dll, msdos.sys, io.sys, pagefile.sys, ntdll.dll, ntdetect.com, hal.dll, and ntldr.
Where Are System Files Stored?
Most Windows computers are configured by default not to display system files in normal file searches or in folder views. This is a good thing — there are very few good reasons to be messing with system files in any way.
System files exist mainly in the Windows folder but can be found in any other places too, like the Program Files folder.
The root folder of the drive Windows is installed to (usually the C drive) has a number of common system files and folders, like hiberfil.sys, swapfile.sys, System Recovery, and System Volume Information.
System files exist in non-Windows operating systems, too, like on PCs with Mac OS or Linux.
How to Show Hidden System Files in Windows
Two things must be done before you can see system files in Windows: 1) show hidden files and folders; 2) show protected operating system files. Both of the aforementioned options are available in the same place, making this process pretty easy.
Before continuing, we must reiterate that there is little if any good reason for the average computer user to enable the display of system files. We only include this information because you may be dealing with a problem in Windows that can only be fixed by accessing a particular system file as part of a troubleshooting process. We highly recommend reversing these steps once you're done working with the one you're after.
There are several ways to show system files in Windows but the following process works equally well in Windows 10, Windows 8, Windows 7, Windows Vista, and Windows XP so we'll go with that route for simplicity's sake:
Open Command Prompt.
Execute control folders.
Tap or click the View tab.
Choose the Show hidden files, folders, and drives option.
Uncheck the Hide protected operating system files option.
Tap or click OK.
See How to Show Hidden Files, Folders, and Drives in Windows if you need more help doing that, or are interested in some of the other ways to go about it.
You may notice that, after performing the steps above, that system files and folders, as well as anything else with the hidden attribute turned on, will be dimmed when they show up in Windows. This is so that you know they are important files that you shouldn't normally see, and not just regular files like documents, music, etc.
More Information on System Files
The system file attribute cannot be toggled on and off as easily as other file attributes like archive files and compressed files can. The attrib command must be used instead.
The system attribute, like any other file attribute, can be manually set on any file or folder of your choosing. This doesn't mean, however, that the data suddenly takes on an important role in the overall function of the operating system.
In other words, if, for example, you save an image file to your computer and then turn the system attribute on for that file, your computer won't crash after you delete this file. It was never was an actual system file, at least not in the sense that it was an integral part of the operating system.
When deleting system files (which we hope you realize by now you shouldn't ever do), Windows will require a confirmation that you really do want to remove it. This is true for actual system files from Windows as well as for files that you've manually toggled the system attribute on for.
While we're on the topic... you normally cannot delete a system file that is actively being used by Windows. This type of file is considered a locked file and won't be able to be changed in any way.
Windows will often store multiple versions of system files. Some are used as backups, while others may be old, previous versions.
It's possible for a computer to become infected with a virus that changes the file attribute of your regular data (non-system files) to ones that have the hidden or system attribute toggled on. If this happens, it's safe to turn off the system or hidden attribute to regain visibility and use the files normally.
System File Checker (SFC) is a tool included in Windows that can repair corrupt system files. Using this tool to replace a system file that's been damaged, or is missing, will often restore a computer back to working order.