Windows photo viewer is showing you the embedded JPEG preview, not the raw image data. Is this problem consistent of all raw images, or just some of them? Have you tried a different card?
Thanks for the answer.
I have Sony RAW Driver für Microsoft Windows. Not all , just some of them. What you mean card? Memory card?
Something between and including the camera and the computer is going bad. Usually it's the card, but it could be the card reader and/or USB-cable, the memory on the computer the hard-disk on the computer, or some other component of the computer.
A way to diagnose what the problem might be is to transfer the images at least twice, and view both sets. If the corruption is identical then it is probably the card or camera. If different data is corrupted between the two sets then it is likely a component of the transfer (reader, cable) or hardware on the computer, itself, that is iffy.
Your camera has a memory card in it which is replaceable. If you have another card I suggest that you try using it in the camera to see if the problem persists. How are you downloading the images from the camera? Are you connecting the camera to the computer, or are you using a card reader? Also, consider what ssprengel has written. I think he's better at these sort of problems than I am.