Video is dimmed after editing

Hey friends,
I have a Samsung 7mega pixel Digital Camera. It also has the video option. Its video quality looks very well in both PC and its own LCD. But the problem is when I edit them and add effects through some video editing software, (e. g. – Ulead Video Studio) its quality becomes too bad.
Furthermore, when I save the video on (*.avi / *.mpg) format it takes too much hard disk space. That is not the major problem because I use 500 gigabyte hard disk. The major problem is the video quality. I have tried every possible setting on the video saving option. But, every times I got same result. But when I import some good formatted files like music videos into the editing software, it results well. So my question is what is the problem with the videos of my Digital Camera’s videos?
Why they become so many odes and dim when I edit them? Is there any other video editing software which keeps the video quality as good as the raw video? I will be very pleased after getting help.
Thank you very much.
