Check the Southbridge, should be a VIA 8235 for USB 2.0, not a VIA 8233. Peter is right in saying that some cases with front USB ports may not be fully capable of the higher speed (though personally I haven't seen a difference, maybe data corruption?) plus many devices don't take advantage of the higher speeds. For instance, most webcams, digital cameras, printers, scanners, joysticks, etc. are all lower speed since they wouldn't be able to use the higher speed. External storage devices, network devices and video devices are the only things I can think of off hand that would even benefit from the higher speed at this time.
WinXP with SP1 supports the higher speed. If your WinXP did not come with SP1 and you had to install SP1 after the initial Windows install, the USB doesn't get updated automatically. AFTER SP1 install you have to go into Device Manager and you'll see a USB device with an error. Double-click on it, and do a driver update. It should automatically find the right driver.
The best way I know of to test for increased transfer speeds is to use a USB 2.0 external hard drive. The difference from USB 1.1 to 2.0 is like night and day. A minute or two per gig of data becomes a half hour on the slower interface.