Also so sollte es gehen:

if (INVALID_HANDLE_VALUE == hCom)
{
Memo1->Lines->Add("Fehler - ");
}
char InString[COM_BUFFER_SIZE + 1];
DWORD dwRead = 0;
ReadFile (hCom, InString, COM_BUFFER_SIZE, &dwRead, NULL);
if (dwRead != 0){
InString[dwRead] = '\0';
Memo1->Lines->Add(InString);
}