You gave precious little info here to adequately answer your question. Are you always using the same USB port, or are you sometimes using one, and at other times using another? I ask this, because my computer has both USB2.0 and USB1.1 ports. Some run at high speed and other are slow. If I plug a device into the slower 1.1 ports, I get the same message as you do. The computer recognizes that the device is a USB2.0 and that it is not using a 2.0 port.
It is also possible that you have a port with a faulty driver, that sometimes does not work as a 2.0. If this is the case you would have to go into the control panel and uninstall the usb drivers, then reboot and allow windows to reinstall the USB ports.
In Xp Start - Control Panel - System - Device Manager
Scan down to find the USB controllers, which will be toward the bottom of the list. Select each controller one at a time and right click. Then click on uninstall. When all are uninstalled, you then can reboot the machine and windows will see the devices and reinstall fresh copies of the drivers.
Test the iPod now and see if the problem is still occurring.