You need new hardware, but a PCI card for USB 2.0 is fairly cheap and
readily available in any major electornics store (e.g., CompUSA or
CircuitCity or many web-stores). I agree with the previous reply that a
combo USB 2.0 and firewire card is a good idea. It uses only only PCI slot
and provides a wider range of connectivity options. For example, most
digital camcorders use firewire, but USB 2.0 is popular for most over
things.
Install SP-1 first. Be sure that the PC is working OK. Then, power-off,
install the PCI card, and power-on. XP should detect the card, load
drivers, and be ready to roll in about a minute. Do NOT install any drivers
provided with the PCI card, unless the manufacture has special intructions
for XP computers that say to do so.