The quick answer is yes - you'll need to install the .Net SDK, I'd recommend getting the latest which is 3.5 from
here. After that, you may be good, but if you were to download
Visual Studio .Net 2008 Express Edition, you'd probably be better.
Also aside from needing to learn ASP.Net, which is a close relative to ASP, but not the same, you'll need to learn a little bit to a lot of C#, but give it some time and understand how everything works together. If you already know ASP, then you should be able to pick up ASP.Net quickly.