Screen Capture in ASP.Net

Hi, I want to capture the current screen in ASP.net, i am using C# as code behind. To capture the screen i used win32 dll's like GetDesktopwindow, GetDC etc. When i build windows application in C# using above said dll's screen capture was working perfectly. But when i try with asp.net the captured image turn to black colour. Please let me know if anyone have idea. Thanks in advance.