I am running Windows XP Professional SP2 on my optiplex 755n. The USB hub shows up as an unknown device (a yellow device with a exclamation mark in the device manager). I've installed most of the relevant drivers from the Dell driver CD coming with the computer. The USB system works but only as USB1, not USB2. When I plug a USB2 device into it, XP tells me that it could be faster if I pluged it into a USB2 port.
Where can I download a driver to enable USB 2.0?
The relevent drivers are part of the operating system; the only time you get special USB related drivers is for the case of a specific peripheral, such as a camera or music player.
What might be the problem is the sequence you used to install the drivers. Dells typically require the Desktop System Software to be installed first, followed by the Intel Chipset Driver, any secondary chipset drivers, video, network (Ethernet), sound, and modem (if present). The Desktop System Software contains tweaks for the os specific to Dells, particularly for the USB ports. I've not heard of a case that the USB 2.0 hardware failed to operate as such when the operator didn't load the Desktop System Software, but it's the only driver I know that has anything to do with the USB ports.