to convert them in to digital images take your negatives to a photo center like walmart, cvs, walgreen, etc. and ask them to make u a cd of those negatives. it will cost from anywhere .29 -.49 cents per image. Most of the time when they convert it to digital images the machine corrects the brightness/contrast. It will be different from your hard copies you already have. Every time you develop a photo form negatives the brightness and contrast will be different from others due to adjustments made during the process.
After you have the digital copy look at them see if you like the results if you still want to enhance the images you can take the cd to a societal photo store and they will enhance the images according to your needs. it could cost you alot or if you have photoshop or any other program you can do it yourself. If you need help you may google it or ask here.
I always enhance my own images using photoshop. I made an album using epson storyteller. Its just a album tht comes with a book, cover and photo paper. you can print your own images after enhancing it. It was a cute little project. I gave her the book on her b'day she loved it because i made it rather getting it done from a photo center :)
How you want to enhance them? do you want to change contrast/brightness? or do you want something like changing the colors, background, adding objects or removing, etc.