rightclick My Computer > Properties > Automatic Updates
Put green dot in 'Notify me but don't automatically download or install them'
click on Apply, click on OK
Now you will receive a pop up to say when updates are waiting to be downloaded and will be able to choose which
When you get another pop up to say they are ready to install, click on Custom Install and take out the tick for IE8 and you will also be asked if you do not want to be notified of that update again, so it will not be in future updates.