Straight from Microsoft Word Help(Word 2000):
Set the default font
If your document already contains text formatted with the properties you want to use, select that text.
On the Format menu, click Font.
Select the options you want to apply to the default font. If you selected text in step 1, the properties you want will appear in the dialog box.
For Help on an option, click the question mark , and then click the option.
Click Default.
Any new document you open will use the font settings you selected.