Prom problems...Girl likes me who's asking me to prom (I don't like her though). How do I let her down gently?

When you confront her don't beat around the bush. You just need to tell her exactly what you just told us. Tell her that you weren't even sure if you were going and tell her that you like her just as a friend but it was sweet of her to ask. Just whatever you do, use your words carefully and make sure not to imply that you don't like her at all or that you have a crush on her.