Iphone Dock Not Charging

Hi guys I got a 3rd party Iphone dock on ebay that claims to work with the Iphone 3GS. I have a 3GS.

I plugged it in and it is acting super funny...here are the facts:

When I plug it in to my Windows XP desktop, 9/10 times it will say that "charging is not supported by this accessory". Once in a while it will begin charging. Weird.

My friend brought his iphone 3G here and it charges in the dock when connected to my desktop.

I connected the dock to my friend's Macbook pro and it charges both of our phones.

If I plug this dock into the wall AC adapter with its provided USB cord, it will charge.

This all makes me think there's something wrong with my computer, maybe the USB current draw isnt letting it charge, but it's all too weird...Any ideas guys?

-OMG