Timeout 시간을 조정해 보세요.
저도 그문제 때문에 한참 고생을 했거든요.
아마 Timeout 문제가 맞을거 같네요.
그럼 .

김용수 님이 쓰신 글 :
: 안녕하세요. 고수님들의 충고 부탁드립니다.
: 아래 소스와 같이 시리얼 통신 프로그램을 작성하여
: DSP와 통신하는 프로그램입니다.그런데
: win98에서는 아주 잘되었는데요, 문제는 win2000에서
: 해보니 프로그램에서 전송은 잘되는데 프로그램으로 받지를 못하고 있습니다.
: ReadFile 부분이 문제인가요?
:
:
: //-------------------------------- main.cpp
:
: hComm = CreateFile(COMPort.c_str(),
:                       GENERIC_READ | GENERIC_WRITE,
:                       0,
:                       NULL,
:                       OPEN_EXISTING,
:                       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
:                       NULL );
:
:     // IF THE PORT CANNOT BE OPENED, BAIL OUT.
:
:     if(hComm == INVALID_HANDLE_VALUE)
:     {
:        ShowMessage("COM Port Open Error, Exit!");
:        Application->Terminate();
:     }
:     else
:     {
:        ComOpenSuccess=10;
:        COMmLED->Color=lcGreen;
:        Com_Ok_Message=COMPort+" Port successfully Opened...";
:        MainForm->StatusBarMain->Panels->Items[0]->Text=Com_Ok_Message;
:     }
:
:    // SET THE COMM TIMEOUTS IN OUR EXAMPLE.
:
:     GetCommTimeouts(hComm,&ctmoOld);
:     /*ctmoNew.ReadTotalTimeoutConstant = 100;
:     ctmoNew.ReadTotalTimeoutMultiplier = 0;
:     ctmoNew.WriteTotalTimeoutMultiplier = 0;
:     ctmoNew.WriteTotalTimeoutConstant = 0;*/
:     ctmoNew.ReadIntervalTimeout = MAXDWORD;
:     ctmoNew.ReadTotalTimeoutConstant = MAXDWORD;
:     ctmoNew.ReadTotalTimeoutMultiplier = MAXDWORD;
:     ctmoNew.WriteTotalTimeoutMultiplier = MAXDWORD;
:     ctmoNew.WriteTotalTimeoutConstant = MAXDWORD;
:     SetCommTimeouts(hComm, &ctmoNew);
:
:     // SET BAUD RATE, PARITY, WORD SIZE, AND STOP BITS.
:     // THERE ARE OTHER WAYS OF DOING SETTING THESE BUT THIS IS THE EASIEST.
:     // IF YOU WANT TO LATER ADD CODE FOR OTHER BAUD RATES, REMEMBER
:     // THAT THE ARGUMENT FOR BuildCommDCB MUST BE A POINTER TO A STRING.
:     // ALSO NOTE THAT BuildCommDCB() DEFAULTS TO NO HANDSHAKING.
:
:     dcbCommPort.DCBlength = sizeof(DCB);
:     GetCommState(hComm, &dcbCommPort);
:     AnsiString BuildComDCBSTRING=IntToStr(COMBps)+","+COMParity+","
:                               +IntToStr(COMBit)+","+IntToStr(COMSTOPBit);
:            ; // "9600,N,8,1"
:     BuildCommDCB(BuildComDCBSTRING.c_str(), &dcbCommPort);
:     SetCommState(hComm, &dcbCommPort);
:
:     // ACTIVATE THE THREAD. THE FALSE ARGUMENT SIMPLY MEANS IT HITS THE
:     // GROUND RUNNING RATHER THAN SUSPENDED.
:     ReadThread = new TRead(false);
:
:
: //(2)-------------------------------- readthread.cpp의 thread에서
:
:  while(!Terminated)
:   {
:     
:          // 데이터를 쓰는 경우의 조건식은 생략합니다
:          TransmitCommChar(hComm,'K');
:
:          // 데이터를 읽는 경우의 조건식은 생략합니다
:           ReadFile(hComm, InBuff, 1, &dwBytesRead, NULL);
:           if(dwBytesRead)
:           {
:             InBuff[dwBytesRead] = 0; // NULL TERMINATE THE STRING
:             Synchronize(DisplayIt);
:           }
:  }