Normally, your scanner should support either TWAIN, WIA or SANE API (or, pretty unlikely, ISIS). Please see:
http://en.wikipedia.org/wiki/TWAIN[
^],
http://en.wikipedia.org/wiki/Windows_Image_Acquisition[
^],
http://en.wikipedia.org/wiki/Scanner_Access_Now_Easy[
^],
http://en.wikipedia.org/wiki/Image_scanner#Applications_Programming_Interface[
^].
Please see the scanner documentation to learn what is supported. A number of Open-source libraries exist to allow you to use the API in C#. Do your search after you learn what is required and can be used with your scanner. When you learn it, do the search using CodeProject, StackOverflow, Google or Bing.
[EDIT]
If forget this is ASP.NET. Dave is absolutely right. You really need to connect a scanner to each client system, for those who have scanners, but a Web application does not have access to the client's computer, by apparent safety reasons. This ActiveX is dirty stuff, only supported by IE and is very unsafe. I would avoid it by any means. I would highly prefer that the users worked with their only scanners by themselves and only uploaded the images. After all, if a user own a scanner, she or he has some software to use it, right? I would hate the idea of having some Web application controlling my scanner, from a standpoint of both user and developer.
—SA