If I understand you properly, you want to change which program opens when a user downloads a file from your page? I don't think you should attempt this. I'm usually pretty easygoing when it comes to what people put on websites but I definately don't want people messing around with my computers registry.
Another meaning to your message might be you want your visitors' browser to display various file formats of your choosing? Browsers can display a variety of files. If they've the plugins for other file formats then the broswer will display/play those too. If they've the programs associated with certain file formats ie Word for .DOC then Word will open the document. If there is a browser plugin for the particular file type then offer your visitors the choice to download it. Again, don't attempt to change your visitors computer settings without giving them some warning of what you're about to do.
The page at http://www.windowsitlibrary.com/Content/368/07/1.html shows how to alter the file associations on your own machine, either by using Windows Explorer or by editing the registry.
