On Windows 7 I plugged in the Kindle into the USB slot and it said I had to eject it if I want to use it during charging. I did it by choosing "safely remove device" from the menu of the usual USB icon in the bottom right corner, but the Kindle kept saying I have to eject it.
Turns out there is an Eject option if you click on Computer in the Start Menu and then right click on the attached Kindle device. When that Eject is selected then the Kindle can be used during charging.
