You should always uninstall the old applications before installing the new
ones (including any activation assistant if it is visible in Add/Remove).
In particular, Office 2007 is very fussy about these things and its security
features will never allow you to install anything over the existing one of
the same type.
Microsoft products these days don't overwrite existing applications so a
careful approach is always required.
Please note Outlook Express cannot be uninstalled or installed unless you
follow specific steps. I don't think you meant Outlook Express but who
knows!
hth