what nic did you put in? maybe you have mechanical problems (does sound like it) but try this:
look in device manager. is the card looking normal, no yellow circled bangs? do you still have the old nic driver installed (control panel/network). remove it. remove both drivers and reboot. put in the new card and driver (again). reboot. works, yes? reboot again. does not work, yes? you may have to go into System Properties/Computer and reserve either the interrrupt or the io base for the thing. try reserving io of 300 to 330h. that's what i had to do for my bargain (?) us robotics 10/100 pcmcia nic.
in my case, i had the old nic driver installed (which shouldn't have been a problem or so i thought). I installed the new nic and booted and it worked. on subsequent reboots, it would not work. reinstalling always made it work - once. in my case, device manager looked normal - at first. once i uninstalled the old nic driver, i got a new symptom, the bang in device manager on the new nic. i explored the nic mfg website and by examining sample settings for dos based installs, i figured out what i/o base to use (i bet 300-330h would work for any nic tho). this problem fix was not documented on the nic mfg website. something about the cards not initialing quick enuf to get their resources allocated or somthing...i found the clues on the mskb. say what you will about microsoft, they have a dang good knowledgebase, hasn't failed me yet in 12 years...