Can't do this from ASP code.
Remember, ASP runs on the *SERVER*. The screen capture would HAVE to happen ON THE CLIENT.
If you intend to do this capture via a browser, then you will need a browser plugin. Probably not the simplest of plugins, and surely very specific to different browsers. Probably have to be an ActiveX plugin for MSIE and a Java plugin for NS. Also, it will surely need very different code for different operating systems (Windows vs. Mac vs. Linux).
In short, it ain't easy.