Windows uses a system called 'file association' to open a file with
the correct program that the file was created in. If a file has an
extension .doc then windows knows that the file is a word document and
uses file association to open the file in the Microsoft Word
application. Or if the file has a .bmp extension then file association
will open the file in Microsoft Paint by default.
All these associations are retained in the registry of the system and
there will be many times that files are downloaded from the internet
and they come with their own third party program to run them. Once we
are done with the files we delete the files but the programs remain in
the system. The list of such programs builds up over time and we need
a windows registry cleaner to clean the registry of these redundant
files and programs.
A file extension is a set of alphabetical characters, sometimes these
are alphanumeric, and these are placed at the end of the file name
after a period. These extensions are also attached to the various
software on the system such as .exe, which means the file is an
executable program. These extensions are used to determine the type of
data stored in them as well as to distinguish the type of program.
This information is recorded in the registry and needs to be cleaned
with a windows registry cleaner regularly....

Registry Repair: http://groups.google.com/group/regrepairsbv