On your PC, click Start, then Run. Type "cmd" into the prompter and press ok (or "command", if you're using Windows 95/98).
Into the command prompt window, type "ping <X-Box's IP>" and press enter. You'll either be told the console replied, or that the attempts timed out.
If the attempts time out:
Try the "ipconfig" command. This'll give you a list of your computer's IP addresses (usually just the one). Don't make the mistake of setting your X-Box to the same address! All network devices in the same LAN need different IPs.
If that doesn't seem to help, list all the IPs, DNS addresses, subnet masks and gateway addresses you're using here.