In the XP installation, go to Control Panel>System>Hardware>Device Manager. In Windows 7 it's just Control PAnel>Device Manager.
Find the entry for either Ethernet Adapter or Network Controller and right click it then click on Properties. Click the Details tab, scroll to HardwareIDs and take a note of the four characters which follow VEN for Vendor and also the four following DEV for Device.
Using anther computer, go to http://www.pcidatabase.com and input those characters in the two relevant boxes and follow the trail to the right driver. When you find the list of the Vendor's drivers, they aren't in order so use your browser's Find facility to track the Device details. With luck you'll have an executable file but some will be a zipped folder.
If it is an executable, transfer it using a flash drive then just click it and let the installation proceed. If a Zip, unzip it then transfer the whole folder then run the Setup file. With a bit of luck, once you have a wired network connection, the Windows 7 machine will be able to find its own drivers by roaming the Net. XP will find some but may need the above treatment for some of the others.