PC won't recognize external usb HDD. I have a 2.5" seagate 100GB that is enclosed in a aluminum

Question

I have a 2.5" seagate 100GB that is enclosed in a aluminum usb HDD case. The connector is a USB Y cable. It is suppose to be compatible with USB 2.0 and USB 1.1. I am running windows xp home on my pc and there is suppose to be no need to install anything for this drive to work. It is suppose to be plug n play but my PC does not recognize it when I plug it in and I have tried all USB ports. When the HDD is plugged in the green light does come on but it is not showing up in my computer nor does the PC recognize it as new hardware. I just purchased it through EBay and it is said to be brand new. What am I doing wrong and how can I fix it or install it so it works.