Do I need to reintall my OS?

My computer is taking a long time to boot up when it is on the desktop. Icons don't show up quickly, and start menu doesn't show up for a long time either. IE 7.0 was also unable to be installed. VERY, VERY frustrated what to do now? Some registry keys, folders and flies have been deleted accidently...Ooops!!!! What would have to be an issue inorder for one to have to reinstall ithe OS?