When you save your invoice document, remember to save it as a Word template, with a .dot file extension. (Go to File | Save As and choose Document Template (*.dot) from the Save As Type drop-down ...

Microsoft Word provides templates for invoice sheets that you customize to suit your business needs. Office features more than 100 online invoice templates that copy to a Word document.

MS Word 2007's invoice templates come in a variety of styles and formats to suit your needs. Click on the Office button, and then click on "New" to create a document that will contain your invoice.