Squares appearing on the screen during fast motion or complex scenes can be caused by the following:

• The way a service provider (cable TV provider) compresses and encodes the video to be sent to thier set-top-box
• The way the video was compressed and encoded on a DVD.

This is also known as macro blocking. An example of macro blocking can be seen in the image below.

Unfortunately this is not a television issue. There is nothing that can be adjusted on the television to prevent this.

If the problem is occuring when watching from a cable box or set-top box, then you need to contact your television service provider to see if they can adjust settings in the signal or set-top box.
If the problem is occuring when watching a DVD, you  may be able to adjust settings on the DVD player to resolve the issue.