How to Convert Excel files into powerpoint

Andrew mentioned below that Office Interop is free, but that's not true, actually far from it. In order to use Microsoft Excel Interop in c# you need to buy and install Microsoft Office. Also you need to have Microsoft Office installed on all machines where you're running your code, otherwise the execution will fail.

Compare to that the standalone 3td party can be a cheaper and a convenient solution. For instance, take a look at these: c# library for Excel and c# library for PowerPoint.
With those libraries you don't need to install and have Microsoft Office. Also there are couple of ways how you could do this conversion, for instance you could read an Excel file in c# and then generate a PPTX file in c# and add a slide with a table element for each sheet in your Excel file.