I found a driver that worked fine with the Windows 7 RC I had installed on my laptop.
Instead of looking for a ethernet adapter driver for the Compaq Presario 2100, I looked for a driver for the device, a National Semiconductor Fast PCI Ethernet Adapter, which I had identified first by using a driver-guide scanner on a similar laptop of a friend.
That way I identified a DP83815 ethernet adapter and then I went to the driverguide.com site and downloaded the driver EN-559TXnew.zip, which worked fine and could start to connect to the internet right away.
Same procedure I used to get a driver for the audio device, a ALi M5451 PCI AC-Link controller, and sound got back to the laptop.
Thank you for your help.