[Paul K]

If you are running IE7 then click on "Tools" at the left end of the bar with the tabs on, select toolbars, customise, click read mail icon and then "add" button. You may now have a little double arrow button at the end of the toolbar which will show the read mail button when you click on them. If you then right click the tab bar/ toolbar and unlock it you can drag it so you can see the icons. (should be a dotted vertical line that you can click on and drag).
If read mail is already in list of buttons to show then all you have to do is resize the toolbar, you may not even have to unlock it if you can already see a vertical dotted line on the toolbar area to the right of your page tabs.
If tools is not at the end of the toolbar then right click an empty section of the toolbar to the right of your page tabs and select "customize command bar", "add remove commands", this will get you to the screen where you add the button again.