If the BIOS is throwing the "stuck key" error, then that probably means that there's a key or keys that is stuck. Fixing it can be as easy as changing the keyboard. Or there may be a problem with the motherboard. You won't know which is the problem until you trying changing one of them and see what happens.
Yes, you can plug in a PCMCIA-to-ethernet adapter, assuming that you also load an OS which supports the card. Windows XP was not offered on the 600E system from the factory so if you use a retail copy to install the OS, then you will have to download the necessary IBM drivers. You can find those drivers here:http://www-307.ibm.com/pc/support/site. ... SHY-3TLQ2L
The Hardware Maintenance Manual for the 600E can be downloaded from here:http://www-307.ibm.com/pc/support/site. ... WIK-3SYPX2