Unknown or New File Types
A file that has no extension or that has an extension that is not listed in the Registry on your computer will need to have some program associated with its type before it can be opened or otherwise used. Some computers are configured to use Notepad for unknown file types but otherwise double-clicking on such a file will bring up the unknown file dialog box (click to see figure) where you can choose to pick a program from a list by choosing the radio button "Select the program from a list" and clicking "OK".
When you choose to select a program, the "Open With" dialog box appears (click to see figure). Windows will list what it thinks are the best possibilities but it is often the case that some other program is what is wanted. Scroll down the list of other programs to get a better selection. Even then, the desired program may not be listed. To see all the program files that are available on your computer, use the "Browse" function. Be watchful of the entry at the bottom of the dialog box, "Always use the selected program to open this kind of file." This is often checked by default and you may permanently assign a program to this kind of file without intending to do so. By leaving the entry unchecked, you can experiment with different programs. However, if you wish to use a particular program for this type of file every time it is double-clicked, place a check in the box.
If you do not know anything about the function of an unknown file, use Quick View (if your computer has it) or Notepad (WordPad for larger files) to see if it is a text or binary file and find any other information that would help to decide if and how it should be opened. References mentioned previously can help you find which software is needed for unknown file types. It may be that your computer is lacking the program needed for a particular file type. For example, if a friend gives you a file that is a spreadsheet created in Microsoft Excel (extension .xls) you will not be able to open the file properly unless you have appropriate software such as Excel itself or an Excel viewer.
A feature in Windows XP is the capability of doing a Web search to find information about an unknown file extension. I have not found it to be useful.