I came across a really old Pentium III computer that mysteriously had trouble with it's network. Despite the hardware being properly detected and installed it couldn't get any IP through DHCP.
Manually renewing the ethernet adapter from command prompt failed with a "The RPC server is unavailable" error. A Google search quickly turned up clues to the culprit: DHCP Client service was disabled.
Head over to Windows XP's MMC and check out under Services. Look for "DHCP Client" and ensure it's set to Automatic. Start the service and your network card should almost immediately successfully obtain a valid IP.