I would try to uninstall Office completely using add/remove programs in the control panel. After that try using a different viewer like openoffice and see if your files will open. You can get it from http://www.openoffice.org/
If you files open with that then they are in tact and you are just having an issue with your ms office trial software. It has a built in window of use so it has probably expired. Reinstallation of the expired trial software won't extend the trial as windows remembers your previous installation.
If openoffice works and you still want to use microsoft, then you should be able to install a full version of ms office, but only if the trial has been uninstalled.