This is usually caused by a registry setting on the client trying to open the XLS file. There is a Microsoft KB article here that explains how to configure a machine to force Microsoft Excel to open the file instead of Internet Explorer.

Because it is controlled by a client-side setting, this behavior cannot be changed from the server. This article will explain how to configure the client machine so that the ExcelWriter-generated file will open in the Internet Explorer browser rather than Microsoft Excel.

Method 1: Use the Folder Options Tool

1. Open My Computer.
2. On the Tools menu (or the View menu), click Folder Options (or click Options).
3. Click the File Types tab.
4. In the Registered file types list, click the specific Office document type (for example, XLS - Microsoft Excel Worksheet), and then click Advanced (or click Edit).
5. In the Edit File Type dialog box, check the Browse in same window check box (or check the Open Web documents in place check box).
6. Click OK.

Method 2: Edit the registry

1. Start Registry Editor (Regedt32.exe).
2. Locate and then click the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes Locate the subkey for the specific Office document type. The following table lists the subkeys for several common Office document types.

   Document Type                                Subkey
   ----------------------------------------------------------------
   Microsoft Excel 7.0 worksheet                Excel.Sheet.5
   Microsoft Excel 97 worksheet                 Excel.Sheet.8
   Microsoft Excel 2000 worksheet               Excel.Sheet.8
   

3. To locate the subkey for a document type that is not included in this table, find the subkey for the extension that is associated with the document type. The (default) value for that subkey contains the name of the subkey for that document type. For example, the .xls extension is associated with Excel worksheets. Under the .xls subkey, the (default) value contains the string "Excel.Sheet.5." Therefore, the subkey for the Microsoft Excel Worksheet document type is the following subkey:
   HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Excel.Sheet.5
4. After you identify the subkey for the specific Office document type, delete the BrowserFlags key.
5. Click OK, and then quit Registry Editor.