Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function IsFileInUse(FileName: TFileName): Boolean;
 var
   HFileRes: HFILE;
 begin
   Result := False;
   if not FileExists(FileName) then Exit;
   HFileRes := CreateFile(PChar(FileName),
                          GENERIC_READ or GENERIC_WRITE,
                          0,
                          nil,
                          OPEN_EXISTING,
                          FILE_ATTRIBUTE_NORMAL,
                          0);
   Result := (HFileRes = INVALID_HANDLE_VALUE);
   if not Result then
     CloseHandle(HFileRes);
 end;


 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if IsFileInUse('c:\Programs\delphi6\bin\delphi32.exe') then
     ShowMessage('File is in use.');
   else
     ShowMessage('File not in use.');
 end;