You can use Windows Script Host to run in a shell other than a browser:
Here’s the Excel 2003 API reference:
UPDATE I forgot to mention that C# is not the only technique to work with Excel files from external code. @KevinCollins is right, there is JScript and VBScript too. For the sake of completeness, I would like to add PowerShell to the list, you can also work with Excel files using PowerShell.