When you double-click a filename in Explorer, Windows uses the filename's extension to make a decision on what to do with it. If Windows "knows" the extension, it will open the file with its associated application. If the extension is not known, a dialogue box will invite you to create a new association. Information about various extensions is maintained in the registry, and you can edit or add to this information using the Windows Explorer. Try the following experiment.

The Win95 Explorer lets you associate file types with the application of your choice

Use Notepad to create a new file called test1.pcw. Open Explorer and double-click the new file. Chances are that Windows has never heard of the PCW extension, so a dialogue will appear prompting you to choose an application that Windows should use to open files of this type. For the purposes of this experiment, select Notepad from the scroll-down list. If you clear the checkbox Always use this program to open this file, you will find that the same dialogue reappears next time you double-click a PCW file. But if you leave it checked, Windows will never raise the matter again, and will always use Notepad to open files of this type. (Sorry, Phil, but I don't know of any way to force this option to appear unchecked by default).
Fortunately, associations like this are not set in concrete. And you will be relieved to know that you can make changes without reinstalling Windows! In Explorer, open the View menu and choose Options--File Types. Scroll through the list and select the new "PCW File" type you created a few moments ago. If you want to make Windows "forget" everything it ever knew about PCW files, just hit the Delete button. Or, more subtly, choose the Edit button and type in the full name and path of an alternative application.
This technique is useful when you want to make semi-permanent changes to file type associations. But it's less helpful when you need to open a file alternately with more than one application. Web pages are a case in point. Sometimes you want to open an HTM file in your browser, and at other times you may want to modify it with a text editor. If you associate the HTM extension with Notepad, you will have to reset the association when you finish editing the file. In situations like this, there's a very handy alternative: select the file in Explorer, hold down the <Shift> key, and click the right mouse button. The resulting pop-up menu will include an Open with option which allows you to open the file with the application of your choice.
- Neville Clarkson