To "format C" means to format the C drive, or the primary partition that Windows or your other operating system is installed on. When you format C, you erase the operating system and other information on the C drive.
However, it's not a straightforward process to format C. You can't format the C drive like you can format another drive in Windows because you're within Windows when you perform the format. To format C from within Windows would be like lifting a chair in the air while sitting on it - you can't do it.
The solution is to format C from outside of Windows, meaning you need a way to format a drive from somewhere other than your Windows installation. The easiest way to do this is to boot to an operating system (with formatting abilities) from a CD/DVD/BD drive, flash drive, or floppy drive.
This might all sound very complicated but it's actually quite easy to format C. Below are 5 completely free ways to format your C drive:
Note: If you're trying to format your C drive because you want to replace or reinstall Windows, you do not need to format C ahead of time. Formatting is done automatically during the Windows installation. Skip this article entirely and instead see How To Clean Install Windows.
Important: Formatting your C drive does not permanently erase the data on the drive. If you want to completely erase the information on the C drive, see Option 5 below, Wipe the Drive Clean With Data Destruction Software.
The easiest way to format C is by completing part of a Windows installation. It's not easiest as far as number of steps goes, but since most of us have a Windows Setup DVD lying around, we have easy access to a means to format drives outside of Windows.
Important: You can only format C this way using a Windows 7 Setup disc or a Windows Vista Setup disc.
However, it doesn't at all matter what Windows operating system is on your C drive, including Windows XP. The only requirement is the disc you use to format C - either a Windows 7 or a Windows Vista Setup DVD. Feel free to borrow a friend's disc if you'd like to try this method but you only have Windows XP... or some other operating system.
If you don't have access to a Windows Setup DVD, but you do still have access to a working copy of Windows 7, you can create a System Repair Disc and then boot to that disc and format C from there.
You can only format C using a System Repair Disc if you have access to Windows 7 to create the System Repair Disc. If you don't have access to Windows 7, find someone who does and create the repair disc from their computer.
Note: A System Repair Disc created from Windows 7 can format a C drive that has any Windows operating system on it, including Windows XP or Windows Vista.
If you have a Windows XP Setup CD, you can format C from the Recovery Console.
The biggest caveat here is that you must also have Windows XP installed on your C drive. However, if you don't have access to Windows 7 or Windows Vista, this option may be your best to format C.
This Recovery Console method to format C also applies to Windows 2000. Recovery Console does not exist in Windows Vista or later, nor does it exist in Windows ME, Windows 98, or earlier.
Several free, bootable, CD/DVD based diagnostic and repair tools exist that have been put together by PC enthusiasts and companies other than Microsoft.
This will be your best option to format C if you don't have access to any kind of Windows Setup disc and can't get access to a Windows 7 PC to create a repair disc.
Any of these tools that have formatting capabilities will be able to format C without a problem.
Note: At the moment, the link above goes directly to the Ultimate Boot CD site, one of several programs that allow formatting from a bootable disc. I'll update this link to a list of such programs soon.
Data destruction software goes a step beyond formatting C. Data destruction software truly destroys the data on a drive, returning it to the essentially the same state it was in after leaving the factory.
If you're wanting to format C because you want to make sure everything on your primary drive is permanently erased, you should wipe your hard drive using these instructions.