Because you elected to upgrade and not parallel install, though you would'nt
have been able to install outlook 2007 if a previous version existed.
By uninstalling the trial and its Activation Assistant, rebooting the PC,
and reinstalling from media your original version
Most people seem to choose the simple automatic install option instead of
Custom. If you choose Custom, you can see exactly what MS proposes to install
(and uninstall) and override those choices if you wish.
If you the "express" (or whatever they call it) install instead of Custom, you
pretty much turn the PC over to MS and say "Take me Bill. I'm yours."