Nothing showing up in device manager is strange indeed - I'd guess it's a messed up system. Set "Plug & Play BIOS present" in BIOS setup to "No" (always works better that way) and check if the NIC shows up on the config status page (may have to be activated, ie. no "quiet boot", "full screen logo" or similar) just previous to booting.
Try to install XP on a spare drive / partition to check whether it's hardware or software related or try a BartPE or Linux live CD.
