I am in my 3rd week of an Office 365 trial ... About a week ago "Wordpad" mysteriously began opening my Office documents. The result is that documents are distorted. How do I turn off Wordpad and return to the normal Office operation?

I have a Dell Latitude E6500. Due to a viral issue I had to reformat my hard drive one month ago. In the process I lost my MS Office 2007 and did not have the old program discs or key to reload it. I was able to save all my documents but couldn't open them. Hence Office 365. Everything then was working well untila few days ago when Wordpad suddenly took over opening my documents.