I connected my iPod to my laptop today and it decharged my iPod until the Apple symbol came on. In stead of seeing the charging symbol, it showed the fully charged symbol while it was connected. This never happened before. Then, I unconnected the iPod from my laptop and connnected the charger directly to the power source and it started charging.
May be your USB cable damaged, just try to borrow a USB cable from friends or relatives to check after changing the cable problem removed or not.
If after changing USB cable it will not get discharged that means your USB cable surely damaged and have to replaced.
I restarted my computer and it is now recharging fine. I don't know what happened. For some reason, at times my USB ports don't charge my iPods unless I restart the computer. I have just never had them decharge an iPod before.
Judging by the fact that it works fine when you restart it, I think Windows is having some difficulty with the USB ports in general.
Charging stuff from a laptop is always bad because the electricity comes from the battery of the laptop and not a real source of energy, this could be the cause.
Try it on another computer you'll see what's wrong (ipod or laptop).
Sometimes windows will not recongnize an iPod, it's done that to me on my old shuffle and my old iPod touch. It might be a matter of updating iTunes, or getting a better computer. I had a machine with P4 and 512mb ram and it wouldn't restore my iPod touch. Now I have a better computer and it works just fine.