The capabilities reported in Device Manager are not necessarily accurate.
Depending on your hardware, some ports may be limited to 100mA while others can do the full 500mA. This is especially common on laptops. If you have USB ports in different locations, try a port from each set.
For example, on my Lenovo I have one high speed port on the left that can do 500mA, while the two slower ports on the right can only do 100mA. (In this specific case, the faster port is actually designed to do double-duty as Firewire with the proper adapter.)
One of my desktop machines in my AV closet has three sets of USB ports and only those on the front panel will do 500mA, although they're all high speed, and they all report 500mA in Device Manager.
It just depends on your hardware. I wouldn't be surprised to hear there are some machines that don't have any 500mA ports.