I had the same issue with the white horizontal lines with myCanon Powershot series camera too. When taking pictures, I get the horizontal lines. In daylight, I almost cannot see the pictures because it gives me very bright output. In darker areas, the picture can still be toleratedbut white lines are still there. Although when I am taking a video,the quality is good.
I was able to read that there is one part insidethe camera's mechanism that is faulty. Its a CCD problem. You should bring this to a CanonAuthorized service center and they should replace and fix the faultypart, free of charge. I got my Camera fixed for FREE! I was able to do macro shots again.Trust me and go to canon. What I did is to print this advisory fromcanon itself. And show it to them.http://www.usa.canon.com/consumer/controller?act=PgComSmModDisplayAct&fcategoryid=221&modelid=8288&keycode=2112&id=29819
As part of their advisory:
"Effective immediately, and regardless of warranty status, Canon willrepair, free of charge, the products listed above exhibiting theabove-mentioned malfunction if Canon determines that the malfunction iscaused by the CCD image sensor. Canon will also cover the cost ofshipping and handling in connection with this repair.
Hope this helps.