Paint

Microsoft Paint, often called "MSPaint" as a reference to its executable's filename, is an application supplied with Microsoft Windows installations. Considered to be the most simple application for artwork, it is perfectly suited for first time sprite creation and simple sprite sheet manipulation. Due to its ease of use and simplistic nature, there is a very common misconception among artists around the web that Paint is somehow inferior to other graphics editors as far as spritework goes. That is simply untrue. Though Paint allows for an easy entry into the worlds of graphics editing and pixel art, it is by no means inferior to other programs from a creation standpoint.

Versions

While later version of Windows supply more complex and feature-rich editions of Paint, it's advised to use to the version released with Windows XP due to its small size, ease of use, portability, and ability to work natively across all Windows OSes prior to XP. (I haven't tested whether or not XP Paint will work with Windows 2000 or 9x releases [I doubt it would run on 9x due to XP being NT architecture but I'll test this out and post an update. ~Vipershark)

There is a website known as "MSPaintXP" that offers a download link of a copy of XP Paint, but the site has locked the copy of Paint in an installer which not only downloads some sort of OTHER installer while it's working, but it pops up several ads in quick succession that are designed to trick the user into installing software other than Paint that the user does not want. The XP version of Paint does NOT and has never needed to be installed on any version of Windows prior to XP, and works will full functionality simply by opening the standalone executable. Because of this shady behavior, I don't recommend or endorse using the MSPaintXP site to get a copy of Paint. Instead, if you have access to a computer with Windows XP, mspaint.exe can be simply copied onto a flash drive or other means of file transfer to allow you to move it to another computer.

A comparison between the two copies of Paint

If you do not have access to a computer with Windows XP, Vipershark has uploaded his personal copy of MSPaint obtained from a freshly installed copy of Windows XP Service Pack 3, which is available for download below:
mspaint.exe on MEGA.(md5 hash: 949bc05cef66bcd68eb23f08eb4c2dff)

The version of Paint that is included in the MSPaintXP installer is actually an older version of Paint and the md5 hash does not match my copy despite their identical file size.

XP Paint

XP Paint, often referred to as "XPaint" when discussing differences between MSPaint releases, is the version of Paint that was included with Windows XP. It is the most commonly used version of Paint and includes a simple toolbar with 16 tools and a palette of 28 colors.