In Windows XP if you wanted to create a new file association for a file type whose file extension wasn't registered, you could do this in Windows Explorer by selecting the Tools menu, Folder Options, File Types, New to open the Create New Extension dialog box.
In Windows 7 there no longer is a way of doing this in the Windows Explorer UI, but the simple workaround is to do the following:
1. Double-click on a file that has the new file extension.
2. Choose the "Select a program from a list of installed programs" option and click OK.
3. Make sure that the "Always use the selected program to open this kind of file" checkbox is selected.
4. Select the appropriate program from the list of displayed programs or click Browse to find the appropriate program and click OK.
As an alternative to the first two steps above you can right-click on a file that has the new file extension, select Properties, and then click Change.