TCP protocol can offload some of the tasks that are being processed by the system processor like Checksum calculation on header and data or IPSec Tunnel Encryptio (if supported by the NIC) onto the Network Interface Card (NIC). Task offloading is designed to improve performance and it is seen to have indeed significantly improve performance.
Windows Server 2008/2003, Windows Vista/XP by defines enables support Task Offloading. However, there may be instance where you need to troubleshoot network performance issues and need to disable the Task offloading. This can be done from the Windows Registry