Optimization - open excel file from VB.NET using interop
Interop is surely enough for this purpose. To manipulate workbooks, fill in 10.000th of cells then I usually use SpreadsheetLight (http://spreadsheetlight.com/), a free, MIT licensed .NET library that works on the XML level of the Excel files and is incredibly fast compared to the Application.Excel object.

From the point of code review, I think you could dive a bit more into VB&#8203;.NET, your code still has the feeling of being VB6 or VBA. Why don't you switch to try/catch error handling for example? I assure you, you are going to like it. And as a rule of thumb, never compile any path/file names into your application, put them into app.config (especially if this is a standalone application) or into start parameters (especially if this application is called from within an enterprise database application).