I need to make this for a friend. Any ideas on how to proceed? I could download the html code, then I don't know how to manipulate it, and then sending it back to the browser (browser class from .Net) and then the user clicks on submit.

Can someone help me on this?

I don't know if i'm clear, english not my main language

Here's the survey : https://docs.google.com/a/cgpvicto.info/forms/d/165MGMHSUt7DHdfP-e0PughQ3pv8HPcORXa2itB4_xp0/viewform

So you want to include this form in to a .Net application that posts this data to google docs?

It's actually pretty easy. All you need to do is make a POST to the correct URL with all of the parameters.

Here's a really good guide explaining the details:
http://www.open-electronics.org/how-send-data-from-arduino-to-google-docs-spreadsheet/

If you use Chrome Developer Tools or Firefox's LiveHTTPHeaders, you can also track the form submit data/URL that you need to replicate in your code as well.