Also another problem that can arise is a Winsock corruption. This can sometimes occur when a new system is set up - especially if you're using Windows 2000. The symptoms are a complete loss of internet connectivity, whereupon local area connections (Settings/local & dial-up connections/local area connection) display packets are being sent but not received.
If you can, get a hold of a file called Winsockfix (windows 2000) or WinsockXPfix (Windows XP). Running same will "usually" clear most network type problems and get you up and running.
Good luck and Si thee.