My Videos From My Digital Camera Appear Grainy When Playing Them At Full Size On My Computer?

When I download video from my digital camera onto my computer, the images appear fine, until I enlarge them to full size. At that point, they become very grainy. Is this because of the quality of video my digital camera takes? Is there any way to correct this on these videos, or future video, or am I best off just buying a video camera? Thank you.