The default MTU in 2000 Pro seems to be "EnablePMTUDiscovery" enabled (value = "1"); I don't think mine has been altered, but I could be wrong.
Discovery lets the OS figure out what the optimal size for the MTU is based on magical testing done in advance of the first ACTUAL data transfer (client session). There is probably a better explanation, a 'certified one', but this is about right.
