A family of Microsoft word processing software products for creating web, email, and print documents.
Different Office versions share assorted resources and files, which makes having both installed problematic as they will constantly battle for control of the registry; and their templates are only 98% compatible which can cause difficulty.
It is not so much a question of having the applications on separate drives (as they are separated anyway) as their working template folders (which are not). These must be separated - see http://www.gmayor.com/Toolbars_in_word_2007.htm and the registry hacked to reduce the conflicts.
Microsoft has complicated the issue further with its click to run installer, which appears to be unstable in many circumstances, but in an ideal world it does allow trial versions to be installed at the same time as it uses a virtual Q drive on which to install the application.
Clearly the installation of the trial has broken the installation of Office 2007. You need to uninstall the trial and repair Office 2007 from the original installation media before you go any further.
If you are going to re-install the trial, if you want it free from problems you must allow it to replace Office 2007, so make sure that you have the 2007 product key for restoring 2007 when the trial runs out.
Note that as you can only have one version of Outlook present, if you install the Outlook 2010 trial, reverting to Outlook 2007 is not without its problems. It will not simply pick up the data and resume from the previous position. You will have to create a new profile, then open the Outlook file from that new profile and then make the file the default data file.
See also http://www.gmayor.com/what_to_do_when_word_crashes.htm