There is something called floorplanner, but I haven't tried it.

Very difficult to see without doing.

Google sketchup allows you to do all sorts of things, I would check that out. People do interior and exterior modeling with it. It is free. There is a large online library of models that people can use.

Hi ! is it just one room you are having trouble imagining or a complete home ?
you could always commission drawings for just the bit you cant get your head around ?
this would not be too expensive ... alternatively sketch up is probably one of the better free ones - also ikea do free planners too ?? and DFS do one for sofa placement that's really easy to use !
best of luck and kind regards,
Karen
OnePlan

http://www.houzz.com/pro/oneplan/oneplan