Maybe this Dell is so new it only has 64 bit Windows 7 drivers. Did you use the dropdown menu to choose an OS? If you report back your Dell tag number I can check with certainty.
If there are no Dell XP drivers for the Gigabit ethernet adapter, then the next step is to Google the exact name of the ethernet adapter + XP driver to find a download for the driver.
If the Device Manager listing for the Ethernet adapter doesn't give you enough information to determine which exact XP driver it uses via Google, then follow these steps to get the exact Gigabit hardware ID to find the driver: Hardware Identification
If you have a wireless adapter to plug in temporarily, you can also search Windows Updates for the driver on the Driver Update tab>Search automatically.
You can also have Device manager>Ethernet adapter>Driver Update> browse for Drivers, then browse to your Windows 7 partition>Windows>System32>Drivers and >Driverstore files to see if the drivers stored there will be picked up and work.
Ask back if you need any further help. This is a process of trial and error which should suceed unless the Gigabit adapter is so new it isn't backwards compatible with XP.