aspx file extension (archive_name.aspx)

ASPX is a file extension used in the latest versions of ASP.NET. It's basically a text file with generally asp code inside.

To open it you can use the well know notepad if you are in windows or any other text editor. Personally I recommend something a little more advance like notepad ++ or any other text editor with code highlighter.

If you are a fan of Macromedia (now Adobe) I recommend you use Dreamweaver but of course it comes with a price tag...

Well I leave the chose to you, follow the force!