Charging an iPad from an old Mac or from a PC seems to be impossible, if you plug the USB cable into the port of your laptop on your iPad you will read "Not Charging". O.o
From the Apple Support site you can find info like the following:
"When connected to lower-power USB ports—those on older Macs, most Windows PCs, and most USB hubs (powered or unpowered)—the iPad’s battery is not charged while the iPad is awake, but is charged (again, slowly) when the iPad is asleep. What’s confusing here is that the message Not charging appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured, Apple says: once you put the iPad to sleep, the battery will indeed charge."
Asus says the Ai Charger works with all motherboards and systems, but because it's hacking the power going through the USB port, this is a use-at-your-own-risk utility and your mileage may vary. After installing Ai Charger on my Dell laptop, my iPad went from "Not Charging" to charging up.
Ai Charger can be found here: http://event.asus.com/mb/2010/ai_charger/
I'm using it on my hp 6730s and it is working fine, but remember use it at your own risk.