My iPad seems not to be charging via USB when I hook it up to my PC ?

The iPad requries a powered USB 2.0 port, since it requires 10 Watts to charge it's battery. Many people have reported though that the device shows "Not Charging" when they connect their iPad to such an USB port.

We discovered that -- although the iPad says different -- it IS actually charging, albeit slower !

Update:

The iPad actually will charge from a normal USB port, but not while the screen is on. If you hit the sleep button it will start charging, you just can't tell because the screen is off. As soon as you turn the screen back on, it says 'not charging' next to the battery. Brings to mind the classic refrigerator light dilemma.