Correction: you need to change your Normal.dot. The .dot is a template file, as opposed to a .doc, which is a document based on a template. All documents are based on templates, and the default is Normal.dot. You can find it with a search. When you get to it, be sure to right click and choose Open, or Open from the File commands in the menu bar. If you simply double-click, you will open a document based on the template.

Once you have the template open, select all (Ctrl+A) and change the columns setting to one. Save the template. You're done.

Does this solve your problem?