Microsoft Office plug-in OfficeTab adds tabbed document support to Excel, Word, and PowerPoint. Microsoft doesn't include this feature by default. You need to download the plaugin.
Once you've installed OfficeTab, Go to the settings panel, where you can enable or disable this application in each application, move the tabs to the side or bottom, and change the appearance of the tabs with a number of styles to choose from. You can right-click the tab bar to add a new document tab, and each time you open a document it will be added to a new tab automatically. OfficeTab works in Office 2003 and 2007 on Windows XP, Vista or 7. It's made by a Chinese developer, so both Chinese and English versions are available.