Excel Services Open in Browser Default
If you create a Report Center site in Sharepoint 2007 and use the Reports Library document library which is created by default, then any Excel workbooks you view from that library are displayed in the Excel Viewer web page by default. Simply clicking on the Excel workbook within the document library opens the Excel document using the Excel Web Access web part in a full web page.
If you are using your own custom library to contain Excel workbooks for viewing via Excel Services however, you won’t get this behavior by default. Instead, when clicking on the workbooks in the document library they will be sent down to the client PC and subsequently opened in the Excel client.
You can still accomplish viewing the workbooks in the web page viewer by clicking the drop down menu associated with the workbook file and choosing View in Web Browser.
This may not be your desired preference though, and you can override this default behavior. The key is in the Advanced Settings section of the document library that houses the workbooks. Select the “Display as a Web page” option here to have the workbooks displayed in the web page viewer by default.