Charge an iPad on MacBook running Win7
Despite the fact that your iPad says it's not charging under Windows, it will in fact charge if the display is off. Check your battery percentage, attach the iPad and turn off the display (standard locking by pressing the power button once); check back in 30 minutes or so and you'll see that you've definitely gained several percent, as much as 10% in that time.

As to why the iPad indicates that it's charging under OS X but not under Windows I cannot say. Fortunately it's lying. :)