import java.awt.*;
import java.awt.event.*;
import java.util.Random;
import javax.swing.*;

public class TicTac implements ActionListener {

  public TicTac(){
    window.setSize(150,150);
    window.setLayout(new GridLayout(3,3));

    // display the buttons, smaller code with loop
    for(int i=0; i<=8; i++){
      buttons[i] = new JButton();
      window.add(buttons[i]);
      buttons[i].addActionListener(this);
    }

    //show the window
    window.setVisible(true);
    }

  private JFrame window = new JFrame("Noughts & Crossess");
  private int count = -1;
  private int[][] winner = new int[][] {
      {0, 1, 2}, {3, 4, 5}, {6, 7, 8},
      {0, 3, 6}, {1, 4, 7}, {2, 5, 8},
      {0, 4, 8}, {2, 4, 6}
    };
  private JButton buttons[] = new JButton[9];
  private String player = "";
  private boolean clicked[] = new boolean[10] ;
  private boolean won = false;

  public void actionPerformed(ActionEvent game) {

    for( int i = 0; i < clicked.length; i++ )
            clicked[i] = false ;

    count++;

    // PC or Human's go
    if(count % 2 == 0){
       player = "O";
       JButton pressedButton = (JButton)game.getSource();
       pressedButton.setText(player);
       pressedButton.setEnabled(false);

    } else {

      Random generator = new Random();
      player = "X";  

       int cpuclick;

      	do {
          cpuclick = generator.nextInt(9); }
    	  while(!buttons[cpuclick].isEnabled());
      	//isEnabled http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Component.html#isEnabled()
      	//this line checks if the button has already been clicked by the computer or human player
      	 {
    	   buttons[cpuclick].setText(player);
           buttons[cpuclick].setEnabled(false);
         }
    	}

     // Is there a winner?
    for(int i=0; i<=7; i++){
      if( buttons[winner[i][0]].getText().equals(buttons[winner[i][1]].getText()) &&
        buttons[winner[i][1]].getText().equals(buttons[winner[i][2]].getText()) &&
        buttons[winner[i][0]].getText() != ""){
        won = true;
      }
    }
    // if there is a winner display who one and close
    if(won == true){
      JOptionPane.showMessageDialog(null, player + " wins the game!");
      System.exit(0);
    } else if(count == 9 && won == false){
      JOptionPane.showMessageDialog(null, "The game was tie!");
      System.exit(0);
    }
  }

  //open TicTac
  public static void main(String[] args){
    new TicTac();
  }

}

Copy of code hosted at http://pastie.org/688030

Code saved as TicTac.java