I’ve recently downloaded Blender 2.71, everything else seems to be working fine except the general undo isn’t working properly also one very strange issue which I have highlited in the screenshot. Whenever I add a new object in the, Blender doesn’t let me change anything initially. The options to make modification is greyed out. Blender 2.70a works perfectly fine.
Enable global undo in user preferences -> editing
Thank you so much. It works now. Although in previous versions I never enabled this option.