Another thing you could try. As your using a plug and play Operating System, let windows configure the devices instead of the BIOS by going into the Bios and selecting the OS as being Plug'n'Play compatible. By default its always set as the OS being non-P'n'P compatible so the BIOS usually set's all the low level settings such as IRQs etc.
It'll be surprising how this can affect problems. It's the only way I can have my WiFi card working, otherwise it shows up also with the same sort of problem.
Just a long shot, as I'm sure you've probably already done this, but make sure that servicepack 1 or 2 is installed, because a clean install of WinXP without a service pack has no USB2 support and if that is the case, it could be causing your problems.