For this you require DCE Autoenhance. you may download either the time limited(30 day) trial version with advanced features or the free version which is more than sufficient for our purpose. link is here:http://www.mediachance.com/digicam/enhancer.htm. after downloading and installing start the programme. load the photo in one window and the enhanced image will be automatically shown on the other. if you are satisfied with the appearance, you can save in various formats. You may change options. But I was satisfied with the default option. There is no need to install and load memory guzzling photoshop. very simple and effective.
Whatever you want to do, keep a backup of your original images.