In my office I have Windows 7 and experienced issues with coping large files (2 to 8 gig) from my Mac, seemed that the NIC card just stopped working. Frustrated I started thinking why it would do this and took a look at the NIC card properties. I turned off Power Management and everything works fine now, speed has improved too.
Yeah, you probably already knew that but I didn't. Seems Windows 7 will engage power management on the NIC even if it is the middle of network traffic.