You can turn off automatic file-type conversions in Word, and instead it will ask you what type of file you're trying to open, at which point you can pick Text.
To turn it off (or rather turn on Confirmation of conversion):
- Open the Word Options. (Word 2007: click the Office button and then click Word Options. Word 2010: display the File tab of the ribbon and then click Options)
- At the left side of the dialog box click Advanced.
- Scroll through the options until you see the General section.
- Make sure the Confirm File Format Conversion On Open check box is selected.
- Click on OK.
If you never want to open rendered HTML in Word again, you can also uninstall the HTML Filter by running Office Setup and de-selecting it form the installed options.
Alternatively and probably easiest: Use something else for the task, like a good Plain Text editor. I personally use and suggest Notepad++.