There are several ways to get back the Word 2000-2003 defaults.
Method 1: Create a new document based on the Office Word 2003 Look template
(Office button | New). Repeat this each time you need a new document with
the look of Word 2000-2003.
Method 2: Set the desired defaults via the Styles pane. To display it, press
Ctrl+Alt+Shift+S. Click the Manage Styles button. Click the Set Defaults
tab. Specify the desired settings. Click "New documents based on this
template." Click OK. If prompted to save the template, choose to do so.
Method 3: Click the Home tab. In the Styles group, click Change Styles |
Style Set, and choose Word 2003. Then click Change Styles | Fonts, choose
Office Classic. Then click Change Styles | Set as default.
Method 4: Change just the line spacing, for example as explained in the
article suggested by garfield-n-odie.