How to charge iPad (v1) on a Windows device
An insufficiently-powered USB port will charge an iPad, just slower than a high-powered one. 

When you plug the USB cable into your laptop the iPad will say "Not Charging." However, when the screen is off it is in fact charging. As soon as you turn the screen on again the battery area will say "Not Charging," too, but again, with the screen off it will charge.

Test it yourself: before plugging in the cable check the battery percentage. Then plug it in and leave the iPad off for 30 minutes; when you turn the screen back on you'll see that the percentage has increased.

In my case it takes several hours to charge from 50% to 100%, but at least it's charging.