IP address won’t renew/something wrong with computer

Ok so obviously when i try to connect to the internet my IP address cennot renew. This is on a LAN line. Not wireless. I called some dell people and they tried to help me but when I brought up the command prompt window, it popped up then disappeared right away. Obviously it is something wrong with the computer, which is a Windows XP desktop, but I have no idea what it is. Please help!!