Installing a PCI card is one of the most basic/ simple things you can do with your computer. Service Pack 1 should include the basic drivers for the USB card to be fully functional after installation. There might be a CD that comes w/ the PCI card - if XP detects the card properly, you won't need to use the drivers on the CD.
After you've inserted the card and boot up the PC, Windows should detect it right away. It will say something like "found new hardware"... look for an icon in your systray (lower right hand corner) that says USB Root Hub. Alternatively, you can look in the device manager to see that the card has been installed properly.
Here are some links on installing a PCI card.