Are you burning DVD Video and do you have AnyDVD installed? Then try turning off AnyDVD before burning.
Are you overclocking your PC? Try disabling the overclocking.
Try burning an ISO/NRG image with ImgBurn (freeware) and see if the verify function in ImgBurn also says that the source and copy are different.
If ImgBurn verification also fails, try replacing the IDE cable to your drive with another 80-wire cable.