This sounds more like a problem with indentation than with margins.
Create a new, blank document. Display the Modify Style dialog box for the
Normal style (which is the style applied to the empty paragraph in new blank
documents by default). Click the Format button, and then click Paragraph.
Under "Indentation," make sure that "Left" and "Right" are both zero and
that "Special" is set to "(none)." Click OK. Before clicking OK to close the
Modify Style dialog box, select the "Add to template" option or "New
documents based on this template" if you are using Word 2007. If/when you
are prompted to save the Normal template, choose to do so.
To display the Modify Style dialog box:
- Word 2003: Click Format | Styles and Formatting. In the Styles and
Formatting pane, right-click Normal and choose Modify from the context menu.
- Word 2007: Press Ctrl+Alt+Shift+S. In the Styles pane, right-click Normal
and choose Modify from the context menu.