There are several ways. If you just want to return to Word 2003's defaults,
in the Home ribbon tab, next to Change Styles, click the dropdown arrow and
choose Style Set - Word 2003. If what you end up with is what you want, then
click the dropdown arrow again and choose Set as Default.
For more granularity, click the Styles launcher (southeast corner of the
Styles chunk in the Home ribbon tab), then click the Manage Styles button
near the bottom of the Styles pane -- Set Defaults tab.
There, you can change the various elements of spacing. Note the option at
the bottom, which lets you apply this only to the current document or to new
documents based on the current template.