>> if(Board[row][cur_col] == Board[row + 1][cur_col- 1] && Board[row + 1][cur_col - 1] == Board[row + 2][cur_col - 2] && Board[row + 2][cur_col - 2] == Board[row + 3][cur_col - 3])


when row == 5 the above causes exception because Board[row+1] is illegal cell reference. I responded with 1 to the question "PlayerO please enter the column in which you would like your marker placed". variable i is hardcoded to = 5 in main(), which may explain the problem.