Comments by HansRios (Top 4 by date)

Wayne suggested using microsoft excel's office interop in c# which I don't believe to be a better choice. I mean if you think about it, with it you're actually running the MS Excel application itself in order to process your files and that is quite an overhead...
Instead I would go with this approach for exporting to Excel with C#.

Do you need a client or a server (note there is a big difference)?
I presume you need a client, because it is a common requirement, so you may want to take a look at this smtp client done in c# and this pop3 client for c#.

I'm not sure what google viewer you're referring to, nevertheless as a suggestion you could convert your powerpoints (both PPTX and PPT) into PDF with C# and then display the resulting PDF to your users.