[not really off-topic] How to register documents to apps on Windows

[Ben Rubinstein]

Can any kind person post a brief run-down of what is required to register a
an extension to an app, so that double-clicking documents with that
extension in the Windows explorer launches the appropriate app?  I assume
that (at least from Windows 98 on) this is done by writing/modifying keys in
the registry - but which and what?  Are there any gotchas to do with
multiple users or Administrator issues in WinXP etc?

TIA,
 
  Ben Rubinstein               |  Email: benr_mc at cogapp.com
  Cognitive Applications Ltd   |  Phone: +44 (0)1273-821600
  http://www.cogapp.com        |  Fax  : +44 (0)1273-728866