You don't say what you see when you right-click on Customers.xsd...
I see a number of options: Open, Cut, Copy, Send To, etc.
When I select "Open", a small "Windows" window opens with two choices: 1) "Use the Web service to find the appropriate program", and 2) "Select the program from a list".
When I select the second option ("Select the program from a list"), and hit "OK", Windows pops up a new window titled "Open With" that includes a list of various application programs including Notepad.
After selecting / highlighting "Notepad" and hitting "OK", the file opens in Notepad.
Note that you may want to uncheck the box next to "Always use the selected program to open this kind of file" or keep it checked depending on how you want these files to be handled in the future.
Also note that there are other ways of getting the file open. For example, the "Send To" option is usually faster than the "Open" option as long as your application is listed. Selecting "Send To" should open a list of applications that include Notepad, which saves you a step compared to the "Open" option.
I hope this helps.