I think the title might explains what i mean .....but what i mean is that i want to capture a whole window........like the part that come when i scroll down also comes.......the whole window . So help plz .
But, re-reading the question, what you want can't be done. You'd need a virtual screen at least as big as the fully displayed web page, I've no idea where you could get a virtual screen from. I've never heard of one.
The thing is that a screen is built in the graphics card memory (or what part of the computer memory you've allocated to graphics, if it's an onboard graphics system). That gives your maximum eg. 1600 horizontal by 1920 vertical pixel limits. On some graphics cards (like ATI Catalyst cards) you can set those maximum values for the card and scroll the screen around it but a screen dump is still limited to the hardware maximum values.
I've looked for a software emulation of graphics memory, thinking maybe you could bully it into giving you a vertical scroll of say 10,000 pixels and fit your full web-page down it, because then a screen capture would show what was in the graphics emulation (it's not limited to the scrolling window on the monitor) but I can't find one, I don't think one exists. Probably because it would be more processor-intensive than you've got processor power to generate. That's why people use graphics cards.
So, since you can't generate a deep enough horizontal and vertical space to display the entire web page (irrespective of the resolution of your monitor), you can't get the image screen-dumped.
I can't think of any other way of rendering it either.