Well go to device manager and see if you even have an ethernet controller installed. You're probably missing all of your drivers. Find out your motherboard model and search the manufacturer website for drivers for USB slots. Or use the setup CD if it's a prefab system, it should have all the drivers on there supposedly.
Do you have an IP address? in start -> run -> cmd -> ipconfig /all
do you see anything or does it just say "Windows XP Configuration". if so, you don't have any network information at all.
If you just got a new HD, like the guy above said, did you put an OS on it, is it your primary drive?