No, I haven't found a copy of garageband for windows, but, i have found something that is near identical to it. However, it is not free, but not extremely expensive either. The program is called Mixcraft, it actually looks and works like a modified copy of the garageband program. If you do not know what garageband is, google it.
Mixcraft is a program developed by Acoustica, it took me about an hour of digging through google to find an artical that mentioned it as an equvilant to garageband. The only bad thing, its not free, but for a mere $50, its worth it if you want a good music composer, quite fun to toy with. Others will probably obtain this by questionable means, but thats there business.
you can download a version to try of Mixcraft here:
http://www.acoustica.com/mixcraft/