Your PIII motherboard most likely supports the older usb 1.1. In order to upgrade to usb 2.0, you will need to install a usb 2.0 pci card. I did this on an old PIII that I own.
It's not at all complicated. Just plug it into a pci slot. Next, install the drivers from the cd that comes with the card. You can find the cards around $20.
You'll see a substantial increase in speed, plus you get extra usb ports.
I have just recently purchased an external CD-RW drive for my laptop (Toshiba Satellite Pro; Pentium 3 with Windows XP Pro for operating system) and the message is telling me that I need a high speed USB port. I did not know there was such a thing. Checked out the USB Man site as recommended in another discussion and I could not make any sense from it. HELP!!!!!!!!!!!!!!!!