Jason Madero has done a post about setting up and publishing Excel Services in MOSS 2007.
His guide will walk you through the steps from configuring your server all the way through publishing and displaying an Excel workbook in the Excel Web Access web part.
Excel services is a Microsoft Office SharePoint Server 2007 (MOSS) feature that can be very effective for a company who is looking to share, secure, and manage Excel 2007 workbooks.
Read his guide at source.