Yup - getting dumped hurts (even if it's just been five days!).
The good - you'll get over it and it relly won't take too long.
The bad - until you do get over it, it hurts like a sum'bich.
The ugly - it'll probably happen again.
The beautiful - the fact that it hurts shows that you CARED! This is a quality sorely lacking by many people. If it didn't hurt, then it meant you hadn't put out any investment, and without that, a relationship isn't worth squat (actually, less than squat).
Sure - it was "only" five days... but any relationship worth having starts off with Day 1, followed closely by Day 2, only to be succeeded by ... well, you get the picture.
How should you treat her now? How would you want her to treat you if, after 5 days, you realized that SHE wasn't right for you and you took the first move? Just 'cause she doesn't want to go out with you doesn't make you or her any different - it just means that things didn't mesh the way she (and you, now) thought they would. That's all.
I HATE the "let's just be friends" line - but in all reality, it's really better than the alternative (i.e "You're an idiot and I'd never, ever want to be seen with you!"). If she was nice enough to go out with, she's OK as a friend. And if people see that you're not some whacko freak, you actually become more, uh, "marketable". She may actually be able to link you up with somone more compatible than she is. Things like this do have a habit of coming around.
This is contrary to some of the advice you'll be getting, but take a moment and look inside... you'll know how to handle it.