If your system doesn't already have USB2 ports installed, the new ports will be added as 1.1 ports I think. You'll need to add a USB2 driver to get them to work at full speed, the link that ORAC posted may contain a suitable driver. As far as I know USB2 drivers are included in the XP service packs, so installing SP1 or SP2 might (cannot be sure!) also solve your problem. If you already have SP2 installed then this is not recommended.
Another option is to look at the card's manufacturer's website to see if they have drivers available.