These days I swiped the Windows installation from a Shuttle XPC system and set it up with a clean install as I plan to give it away. While I was cleaning the mainboard from dust, I found the single SRAM module to be sitting in the slot DIMM1. On most boards a single stick of RAM is expected to be put in slot #0, so I swapped the module over to the first connector. I then updated the BIOS and installed Windows XP SP2, but not without difficulties. The installation would not finish with an AGP graphics card connected, and even after I removed it the network adapter was neither recognized by Windows nor a Linux system. Downgrading the BIOS and clearing the EEPROM did not help, so in an desperate attempt to restore the LAN adapter I plugged the RAM module once again in slot #1. And you know what, Windows boots up and recognizes the adapter just fine. This is not mentioned in the mainboard manual and installation guide so I had to find out the hard way. So if your Shuttle FS51 v2.0 (aka SS51G) acts weird try to change the RAM slots.
Update 25.08.2011 Well, as it turned out the RAM wasn't the problem. After I reinstalled Windows XP on the machine the network card did not show up in the device manager. In a desperate attempt to restore the network connection I measured the BIOS backup battery and voilà, it was empty (down to 1.5V). I replaced it with a fresh one and the network card magically appeared in the device manager. Everything seems to be back to normal now so I can finally finish the installation process.
Update 27.08.2011 The network adapter disappeared again! A CMOS clear brought it back for some time, but then it disappeared again. The CMOS clear should be done after every BIOS update by moving the on-board jumper to its second position and back after several seconds - lesson learned. After checking the power supply (all good), the caps on the mainboard (also good) and a lot of hair tearing I finally came back to the BIOS settings. I hadn't changed the "Reset Configuration Data" switch before so I gave it a try. And the network adapter finally came back. It even stayed after I changed back the switch to Disabled. I don't now how long it will take until my RTL8139 goes back to its happy place but for now I'm confident that I solved the problem. I read about the meaning of this BIOS setting and what the Extended System Configuration Data (ESCD) is, and it seems plausible that this was the problem. Time will tell...
Update 28.08.2011 I give up. The network card has been and gone. Under the screen of night it left and I have been unable to bring it back. From the looks of it something is seriously messed up with this PC, but I won't bother even further.
R.I.P. RTL8139 2003 - 2011