Can’t load Fiddler because Fiddler Port in Use

you should use

DWORD GetExtendedTcpTable (PVOID pTcpTable,PDWORD pdwSize, BOOL bOrder, ULONG ulAf, TCP_TABLE_CLASS TableClass,ULONG Reserved );

with

TableClass value = TCP_TABLE_OWNER_PID_ALL "or" TCP_TABLE_OWNER_PID_CONNECTIONS "or" TCP_TABLE_OWNER_PID_LISTENER

pTcpTable structure -> MIB_TCPTABLE_OWNER_PID

depending on the info you'd like to retrieve

EDIT:

TCP_TABLE_OWNER_PID_ALL returns MIB_TCPTABLE_OWNER_PID structure that is an array of MIB_TCPROW_OWNER_PID structures where each dwState should have MIB_TCP_STATE_CLOSED when bound and not listening, this structure also offers dwLocalAddr and dwLocalPort

typedef struct _MIB_TCPROW_OWNER_PID {
  DWORD dwState;
  DWORD dwLocalAddr;
  DWORD dwLocalPort;
  DWORD dwRemoteAddr;
  DWORD dwRemotePort;
  DWORD dwOwningPid;
} MIB_TCPROW_OWNER_PID, *PMIB_TCPROW_OWNER_PID;

Try looking in IIS manager, right-clicking on the default web site and selecting Edit Bindings. Check that you don't have any extra/blank bindings to port 80. You may need to remove an empty binding.