Pictures and Videos Look Overly Pixelated when Enlarged

Hello.
I have some problems viewing pictures and videos in my computer. I used the default Windows picture viewing in opening pictures.
It opened but the problem is when I enlarge it. It looks overly pixilated and I know it happens when stretched but this is very different. It’s so pixilated to the point that it’s hard for me to recognize the picture.
I installed another program which is Picasa viewing picture. I opened the picture using it and then I zoom in the picture for over 200% but it has smoother pixilation.
So, Picasa worked fine.
Now, I also experience the same problem with videos. I installed Real player and it is the one I used to open the videos.
When I enlarge the video, it was so pixilated.
What should I do?
Do I have to find another video player?
Can someone explain to me what’s causing this?
