If you are looking for a free program to calculate and verify the MD5 checksum of a file, then WinMD5Sum is for you, it’s a lightweight and easy to use freeware, and works on Windows XP / Windows 7.
After you’ve downloaded and installed it, you can click the file to checksum with right button, then choose send to WinMD5Sum, then WinMD5Sum will pop up and calculate the MD5 hash value for you automatically, then you can paste the MD5 hash value copied from the product web page, and press the Compare button to verify it.
You can also launch WinMD5Sum first, then drag and drop file from Windows Explorer to WinMD5Sum.
WinMD5Sum supports the following operating systems: Microsoft Windows 7 / Windows XP / 95/ 98 /Me / 2000.
What is MD5?
In cryptography, MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value. Specified in RFC 1321, MD5 has been employed in a wide variety of security applications, and is also commonly used to check the integrity of files. However, it has been shown that MD5 is not collision resistant; as such, MD5 is not suitable for applications like SSL certificates or digital signatures that rely on this property. An MD5 hash is typically expressed as a 32-digit hexadecimal number.( source : http://en.wikipedia.org )