This is the line spacing setting in the Style that is being used, then. Modify the Style you use or create a new Style to use. I use Body Text for most of my work.

If you have Office XP you are working with Word 2002.

Create a new blank document. Under the Format menu, select the "Styles and Formatting" choice. A list of styles should pop up with the one in use highlighted in some way. Right-click on that and choose modify. A dialog box will pop up. On that box you want the checkbox to "add to template" checked and any box about automatic updating unchecked.

Click the Formatting button in the lower left corner and choose "Paragraph." That box has the setting for line spacing. Set it for single spacing and then OK your way out. Close the document and Word. If a question comes up about saving changes to the normal template, answer yes.

Restart Word and see if your change stuck. If not, write back.

Except reading your original post about .dotx files, you can't be using Office XP!

Which version are you using?

Have you already set up your defaults in the normal template?
Normal Template in Microsoft Word - How to Open