The majority of PCs are unable to charge the Apple device when plugged into a USB port, while all Macs (since mid-2007) are able to do so.
The Apple device requires more than the usual 500 mA (at 5V) of USB ports and will only charge at 1 A. Contrary to popular belief, Apple does not use a proprietary system, but a USB standard options ("Battery Charging").
Specifically, the standard indicates that a classic USB 2.0 port can supply 1.5 A to the compatible devices and a dedicated USB port for recharging may even offer up to 1.8 A.
Here's a solution from Azus:
A software named Ai Charger that can deliver 1.2A instead of 0.5A to charge the iPad!