I ran into an interesting problem today where I had a SharePoint site setup with anonymous access permissions and I was getting reports that users could not download XLSX files from this site. After doing some digging I found that Excel Services for SharePoint was trying to launch the document as the default application instead of Excel from the desktop. This site showed me how to disable this functionality for XLSX files.
SharePoint Prompts Login when Opening a XLSX file on a Publishing SiteFebruary 2, 2010
Edit the file "serverfilesExcelServer.xml" under C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\XML
Comment out this line:
<Mapping FileExtension="xlsx" RedirectUrlTemplate= "/_layouts/xlviewer.aspx?id=|0″ NoGetRedirect="TRUE"/>