If your EHD is USB than that is what it is - Firewire is a different technology. as stated a USB 2 PCI card is the answer -quite cheap less than a tenner at a computer fair or even your local independant shop. PCW will wanr more -of course Less by mail order but postage on top.
The Belkin USB2 PCI Card that I bought last year from PCW, included a CD with drivers for non-XP configured PC's. The PC I fitted the card into had XP so did not load the drivers, therefore cannot comment if OK or not.