What kind of nic is it? Intel cards used to come with a management tool that let you look at what the speed and duplex were actually running at (as opposed to "auto negotiate").
You could try going into the nic properties and click Configure, then find the speed and duplex setting and hard code it at 1gb, full duplex and see if that changes anything.
Or you could uninstall the device, then reboot to have Windows install it fresh and see if that makes a difference.
If none of that helps, go ahead and run
TCP Optimizer and have it set up reasonable values for the network stack. Something may have gotten honkled in there.