SharpPcap

Riyaz,

Closing a connection by sending a spoofed RST pkt is not a trivial task. You need to keep track of the sequence numbers of the connection and inject your packet with the correct sequence number expected by the target machine. With some luck you may even succeed with this.
However, most probably this is *not* the way sysinternals had implemented their tcpview application. They probably utilizing some Win32 API call to terminate an existing socket, which causes the Windows TCP/IP stack to send a legit RST packet.