Here's the code:

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
const char X = 'X';
const char O = 'O';
const char EMPTY = ' ';
const char TIE = 'T';
const char NO_ONE = 'N';

void instruction();
char askYesNo(string question);

int askNumber(string question, int high, int low = 0);
char humanPiece();
char opponent(char piece);
void displayBoard(const vector<char>& board);
char winner(const vector<char>& board);
bool isLegal(const vector<char>& board, int move);
int humanMove(const vector<char>& board, char human);
int computerMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);

int main()
{
int move;
const int NUM_SQUARES = 9;
vector<char> board(NUM_SQUARES, EMPTY);

instruction();
char human = humanPiece();
char computer = opponent(human);
char turn = X;
displayBoard(board);

while (winner(board) == NO_ONE)
{
if (turn == human)
{
move = humanMove(board, human);
board[move] = human;
}
else
{
move = computerMove(board, computer);
board[move] = computer;

}
displayBoard(board);
turn = opponent(turn);
}

announceWinner(winner(board), computer, human);

return 0;
}

void instruction()
{
cout << "Welcome to the ultimate man-machine showdown: Tic-Tac-Toe.\n";
cout << "--where human brain is pit against silicon processor\n\n";

cout << "Make your move known by entering a number, 0 - 8. The number\n";
cout "corresponds to the desired board position, as illustrated:\n\n";

cout << " 0 | 1 | 2\n";
cout << " ---------\n";
cout << " 3 | 4 | 5\n";
cout << " ---------\n";
cout << " 6 | 7 | 8\n";

cout << "Prepare yourself, human. The battle is about to begin.\n\n";
}

char askYesNo(string question)
{
char response;
do
{
cout << question << " (y/n: ";
cin >> response;
}while (response != 'y' && response != 'n');

return response;
}

int askNumber(string question, int high, int low)
{
int number;
do
{
cout << question << " (" << low << " - " << high << "): ";
cin >> number;
}while (number > high || number < low);

return number;
}

char humanPiece()
{
char go_first = askYesNo("Do you require the first move?");
if (go_first == 'y')
{
cout << "\nThen take the first move. You will need it.\n";
return X;
}
else
{
cout << "\nYour bravery will be your undoing... I will go first.\n";
return 0;
}
}

char opponent(char piece)
{
if (piece == X)
{
return 0;
}
else
{
return X;
}
}

void displayBoard(const vector<char>& board)
{
cout << "\n\t" << board[0] << " | " << board[1] << " | " << board[2];
cout << "\n\t" << "-------";
cout << "\n\t" << board[3] << " | " << board[4] << " | " << board[5];
cout << "\n\t" << "-------";
cout << "\n\t" << board[6] << " | " << board[7] << " | " << board[8];
cout << "\n\n";
}

char winner(const vector<char>& board)
{
const int WINNING_ROWS[8][3] = { {0, 1, 2},
{3, 4, 5},
{6, 7, 8},
{0, 3, 6},
{1, 4, 7},
{2, 5, 8},
{0, 4, 8},
{2, 4, 6} };

const int TOTAL_ROWS = 8;

for(int row = 0; row < TOTAL_ROWS; ++row)
{
if ( (board[WINNING_ROWS[row][0]] != EMPTY) &&
(board[WINNING_ROWS[row][0]] == board[WINNING_ROWS[row][1]]) &&
(board[WINNING_ROWS[row][1]] == board[WINNING_ROWS[row][2]]) )
{
return board[WINNING_ROWS[row][0]];
}
}

if (count(board.begin(), board.end(), EMPTY) == 0)
return TIE;

return NO_ONE;
}

inline bool isLegal(int move, const vector<char>& board)
{
return (board[move] == EMPTY);
}

int humanMove(const vector<char>& board, char human)
{
int move = askNumber("Where will you move?", (board.size() - 1));
while (!isLegal(move, board))
{
cout << "\nThat square is already occupied, foolish human.\n";
move = askNumber("Where will you move?", (board.size() - 1));
}
cout << "Fine...\n";

return move;
}

int computerMove(vector<char> board, char computer)
{
unsigned int move = 0;
bool found = false;

while (!found && move < board.size())
{
if(isLegal(move, board))
{
board[move] = computer;
found = winner(board) == computer;
board[move] = EMPTY;
}

if (!found)
{
++move;
}
}

if(!found)
{
move = 0;
char human = opponent(computer);

while (!found && move < board.size())
{
if(isLegal(move, board))
{
board[move] = human;
found = winner(board) == human;
board[move] = EMPTY;
}

if(!found)
{
++move;
}
}
}

if(!found)
{
move = 0;
unsigned int i = 0;
const int BEST_MOVES[] = {4, 0, 2, 6, 8, 1, 3, 5, 7};

while (!found && i < board.size())
{
move = BEST_MOVES[i];
if (isLegal(move, board))
{
found = true;
}

++i;
}
}

cout << "I shall take square number " << move << endl;
return move;
}

void announceWinner(char winner, char computer, char human)
{
if (winner == computer)
{
cout << winner << "'s won!\n";
cout << "As I predicted, human, I am triumphant once more -- proof\n";
cout << "that computers are superior to humans in all regards.\n";
}

else if (winner == human)
{
cout << winner << "'s won!\n";
cout << "No, no! It cannot be! Somehow you tricked me, human.\n";
cout << "But never again! I, the computer, so swear it!\n";
}

else
{
cout << "It's a tie.\n";
cout << "You were most lucky, human, and somehow managed to tie me.\n";
cout << "Celebrate... for this is the best you will ever achieve.\n";
}
}

Thanks for the help in advance!