I have a brand new arduino(original), and it is not showing up in Device manager or anywhere else. Even the computer doesn't Prompt anything. I am using Windows 7 32 bit.
As the OP reported: It worked with a different cable.
I converted his comment to an answer so the system would regard this issue as solved. Please accept this answer.
Some cheap USB cables only carry power, and not data. I have such a cable that came with a USB desk light. To save money they only run two wires and not four, since the desk light is not a data device.
Be cautious about grabbing any old cable from your desk for testing. Make sure it is designed for data use. If in doubt, try a known good cable.