Email viewing

When looking at my inbox I try to click an email to view it's content. It use to open a half tab and I could view the content of the highlighted email on one side of the half tab and my full inbox on the other side of the half tab. Now when I click on an email the half tab shows for about a second then disappears. It works on all computer except my primary office computer running Apple OS 10.6. Could you offer any insight as to what steps I could take to fix this situation? Thanks!!

Craig