C# multithreading problem

Hi, I've just started playing around with C# multithreading and I've encountered some strange behaviour. I have two procedures, each of which does some integer arithmentic and takes about 3 seconds to execute on a P4:

private volatile int intResult1 = 0;
public void WorkerThreadProc1()
{
  for (int i = 0; i < 10000; i++)
    for (int j = 0; j < 100000; j++)
        intResult1 += i * j;
}

private volatile int intResult2 = 0;
public void WorkerThreadProc2()
{
    for (int i = 0; i < 10000; i++)
        for (int j = 0; j < 100000; j++)
            intResult2 += i * j;
}

And I'm creating a seperate execution thread for each:

Thread thread1 = new Thread(new ThreadStart(WorkerThreadProc1));
Thread thread2 = new Thread(new ThreadStart(WorkerThreadProc2));

Now if I execute these threads sequentiallly then it takes 6 seconds to run, as expected:

thread1.Start();
thread1.Join();
thread2.Start();
thread2.Join();

However, if I switch two of those lines to try to get them to execute concurrently then it takes a whopping 34 seconds:

thread1.Start();
thread2.Start();
thread1.Join();
thread2.Join();

The point of all this is to try to get the routines to execute concurrently if the user has a core2 duo or whatever, what am I doing wrong? I understand there'll be some overhead from multithreading on a single core CPU, but 90% of CPU time is ridiculous.