Update: Debian Lenny 5.02 Installer and Intel D945GSEJT
When I installed Debian Lenny on my Intel D945GSEJT home server, I reported that the Debian installer did not recognize the network adapter. With the latest version of the installer (5.02), the r8169 module is loaded, and at least on my system, DHCP autoconfiguration of the network connection succeeds. This is a step up from the previous installer, but r8169 is in fact the wrong kernel module, as reported in this post. What you want is the r8168 module, and you can find instructions on compiling that module in the previous link.