The files may be corrupted, and the question is to determine where in the process that is happening.
Retransfer the files to your computer and see if the exact same lines appear or if they are in different locations or different photos have the lines.
Sometimes it can be the card, or cable, or computer memory or computer hard-disk that is failing.
If you want you can upload one of your files to http://www.dropbox.com/ and post a public download link to it, here, then others can test to see if the same corruption happens for them. If not then it is likely your computer memory, if so then the data in the file is actually bad and you can try swapping out various components used to transfer the photos from your camera to your computer and see if anything changes. Usually it is a bad card, though, so that’s the first thing to try a new one of. Secondly, if you’re using a cable to connect directly to the camera, then try using a card reader, instead.
It’s also possible that something else is happening that is not obvious from your description, so posting a screenshot in a message, here, can help with the communication of the issue.