I agree it's a copout to install Windows just because you don't know how to use Leopard
I disagree. I've been using windows for 18 years. I've also used MVS, OS/2, OS/400, DOS, Sun Unix, various Linux variants, various Unix/Linux shells, and MAC/OS.
I use windows because I am a software developer in a corporate world developing apps using Microsoft tools. However I still have a choice, and I choose to use windows because I prefer it to MAC/OS.
Now, please don't flame me. I appreciate that I'm in a Mac forum, and I don't really expect to anyone to agree with me. But, please accept that there are people out there who make different choices. Because their choices are based on different criteria, and as such come out different to your choice, that doesn't make them the wrong choice. I'm not putting anyone down for choosing MAC/OS or any other OS for that matter - if it's right for them, then who am I to comment.
just a waste to buy a MBA just to use Windows
Again I disagree. Why did you buy an Air? You had a choice, but you chose the air because it met your requirements. Which I'm guessing were for a small, light, portable, powerful, stylish computer which doesn't sacrifice a usable keyboard or screen.
Those are exactly my criteria, and the air meets them. What's even better is that Apple has the foresight to allow me to choose windows if I wish, and even actively supports that choice by providing the bootcamp drivers.
I ordered an Air on the day I first saw it, and have been running Vista for 2 and a half weeks on it. I love it because it does everything my old Sony laptop did, but it's presented in a much better package with better screen, keyboard, and performance.
How is that a waste?
As for OPs original question, a laptop will consume a small amount of power, even when in standby. If you're on low battery, and leave it overnite it may drain the remaining battery which will cause your problem.
Are you connecting the power overnite? If not, give it a try and see if it makes a difference.