MD5 Sums are 32 bit strings that are the result of running the
MD5 sum program against a particular file. Since any difference
between two files results in two different strings, MD5's can
be used to determine that the file or iso you downloaded is a
bit-for-bit copy of the remote file or iso.

Often in the FTP directory of a ISO to be downloaded there is
a small MD5xxxxxx text type of file (ie. listed md5 sum), so
if you download it and open it with a text editor you will see
what the MD5 sum should be after you have downloaded the ISO.

If you are running one of the GNU/Linux distributions, you
should already have the MD5 program installed. If you run Windows
you can use this MD5SUM.EXE program. After running your preferred
anti-virus software on this file (to ensure its still virus free),
copy or move it to your c:\windows\command directory. Then, open
up an MS-DOS window, and go to the directory of the downloaded
iso file that you wish to check. Once you are in that directory,
type: md5sum xxxxxxxx.iso

TAKE NOTE:
Since this is being done in dos, the eight character file name
limit applies. To get around this, rename the .iso file to an
eight character name similar to the original file name. Once the
program has run, and it will take a few minutes to run on a
640 megabyte file, a 32 digit md5sum will be generated. This sum
should be exactly the same as the listed md5 sum for the specific
downloaded iso. If the sums are different, your downloaded iso
is not an exact copy, and should be downloaded again.


------------------------

Where to find MD5SUM.EXE: you can have a look at 

 http://linuxiso.org/md5sum.html

 ftp://www.toast442.org/pub/md5.exe

 http://etree.org/md5com.html 

 http://www.edv.agrar.tu-muenchen.de/~syring/win32/UnxUtils.html 

 http://members.tripod.com/~rimeswel/shnguide.html 

 http://home.earthlink.net/~mzupanek/dae.htm 

Many thanks to John Coombes for pointing out these links
to us.