1. Your panorama is not completely level. This can be frustrating to deal with, but you will need to take time to carefully level your tripod as this bothers people. You can also use a bubble level that is placed on your camera hotshoe for external flash units.
2. In the Ptgui stitcher you use vertical and or horizontal control points. You draw two points along something straight such as the side of a building. Ptgui will then straighten the panorama.
3. In the viewer you may limit the FOV ( field of view ) to a lower value to decrease the warping.