Cannot view image. get message "cannot playback image"
There are four possible reasons that this could happen, so you need to do some tests to diagnose which one it is.
1. It could be that you shot some photos in "RAW" and the photo viewer is not capable of decoding raw images. (You should have a digtal developing software such as LightRoom for this, or installing the right Canon software might include the decoder that allows other programs to access it.) To test for this, check the full name of the files; if you can read all the .JPG files, but none of the .CR2 files, this is your problem.
2. It could be that your card is corrupting the data. To check for this, try a different card. (Any card will work for photos.)
3. Failing the other two, either the camera is producing corrupted data, or your image viewer is not capable of reading them. To discern which, try a different image viewer. If you don't want to install one, upload it to a photo sharing site, or just view it in your browser (you can drag and drop).