You can get hard drives that operate on USB, but they will be quite slow compared to firewire (which I use) or USB 2. Looking in the device manager dialogue box in the System part of Control Panel should tell you what the USB ports you have are.
It depends what you want to use the drive for, as USB 1 is substantially slower. I use mine purely for back up so speed isn't that important to me, although as it's firewire it isn't an issue anyway. You can buy a firewire card for around £35, and a USB 2 one for around the same. If your computer is 2 years old it's probably got USB1.