there's a bunch of things to troubleshoot, slow network traffic is almost always the physical network, cables, switch, or the NICs.
get a crossover cable and hook the machines straight together, skip the router/switch
swap the switch.
swap the patch cords.
get another machine to test speed with a known good system.
try browsing by IP instead of name or network neighborhood.
The MB/Driver thing is certainly a concern, but I'd expect BSoD or Freezes or bizarre behavior. not really working but slow Network traffic.