Here is a quick instruction on how to install the Microsoft Excel 2007 and Microsoft Word 2007 default icons to your SharePoint 2003 site.
First of all you need these two icons, right-click them and choose Save Picture As.
Copy the two .gif files into the C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\IMAGES\ folder.
Then open the file C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\XML\docicon.xml with your favorite XML editor and add these two lines under the ByExtension element:
<Mapping OpenControl="SharePoint.OpenDocuments" EditText="Microsoft Office Word 2007" Value="icdocx.gif" Key="docx" /><Mapping OpenControl="SharePoint.OpenDocuments" EditText="Microsoft Office Excel 2007" Value="icxlsx.gif" Key="xlsx" />
Save the file and reset the IIS and you’re done.