In order to change to a single-column layout in an existing document,
open the document and then click Columns on the Format menu. Click the
"One" option. Make sure that "Apply to" is what you want, and then
click OK. Save the document.

A similar technique can be used to modify the default. Open the normal
template as a document, via File>Open (for details, see
http://word.mvps.org/FAQs/AppErrors/BlankDocNotBlank.htm). On the
Format menu, click Columns. Click the "One" option, make sure "Apply
to" is set to "Whole document" and click OK. Save and close the file.

For a quicker fix, you can rename the normal template as discussed in
the section titled "Creating a new Normal.dot from scratch" of the
article at http://word.mvps.org/FAQs/AppErrors/BlankDocNotBlank.htm.