Tic-Tac-Toe help

I am making a basic Player vs. Computer Tic-Tac-Toe in Visual Studios Express 2008. My Form.cs is

Code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Tic_Tac_Toe
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public Image x = Image.FromFile("X.png");
        public Image y = Image.FromFile("Y.jpg");
        public Boolean pTur = true;

        private void Form1_Load(object sender, EventArgs e)
        {
            int n;
            pictureBox1 = new PictureBox[9];
            pictureBox1[0] = pictureBox2;
            pictureBox1[1] = pictureBox3;
            pictureBox1[2] = pictureBox4;
            pictureBox1[3] = pictureBox5;
            pictureBox1[4] = pictureBox6;
            pictureBox1[5] = pictureBox7;
            pictureBox1[6] = pictureBox8;
            pictureBox1[7] = pictureBox9;
            pictureBox1[8] = pictureBox10;
            for (n = 0; n < 9; n++)
            {
                this.pictureBox1[n].Click += new System.EventHandler(this.oPc);
            }


        }
        private void oPc(object sender, EventArgs e)
        {
            PictureBox sen = (PictureBox)sender;
            if (pTur == true)
            {
                if (sen.BackgroundImage == null)
                {
                    sen.BackgroundImage = x;
                    pTur = false;
                    //xOne();
                    cPc();
                }
            }
        }
        void cPc()
        {
            Random trytg = new Random();
            int trytg1 = trytg.Next(0, 9);
            Boolean r = true;
            while (r == true)
            {
                if (pictureBox1[trytg1].BackgroundImage == null)
                {
                    pictureBox1[trytg1].BackgroundImage = y;
                    pTur = true;
                    r = false;
                }

            }
        }

        public Boolean hasX(PictureBox sende)
        {

            if (sende.BackgroundImage == x)
            {
                return true;
            }
            else
            {
                return false;
            }

            
        }
        public Boolean hasY(PictureBox sende)
        {

            if (sende.BackgroundImage == y)
            {
                return true;
            }
            else
            {
                return false;
            }
        }
    }
}

Sometimes, it just freezes when I click on a cell and "thinks" forever. My computer is exceptionally fast, so what is the problem... maybe random numbers? Please help quickly