I have found the lower end Cannon lenses to exhibit noticeable focal fall-off, a degradation in focus as you move out from the center of the lens. 
If the center portion of your photos are acceptably sharp and progressively blurred toward the edges this is the problem. The effect is most noticeable in wide angle shots.
Autofocus can have difficulties determining focus in certain situations. The program has to have a clean edge to focus on. Have you tried a comparision test on a subject using auto and manual focus?
The advice by Inshaala is spot on, fixed focal length lenses will give you the sharpest results.
Also the first thing I do with anydigital photo is use a light unsharp mask filter in my image processing application to account for the digital format effect Inshaala mentioned. Unless I'm looking for a soft focus then I skip that step.
Try the test I mentioned and unsharp mask filtering in post processing. If your results are negative then post back and I can go through a few other things with you.