It is best to fully remove the evaluation copy MS Office.
A friend had problems with the install as well with an older version while the eval remained hidden on the PC.
After suggesting he run Revo Uninstaller to completely remove the trialware he was able to successfully install the older Office suite.
The free version will do the job for you:http://www.revouninstaller.com/revo_uninstaller_free_download.html
The symptoms weren't the same but this fixed it.