One way to do this:
select a file, let's say a .doc (one that you know that you know should open with word) ctrl-click on it, select open with -> other... a window will pop up, select Word from the suggested apps, and don't forget to check the "always open with..." . From now own your default app will be Word for all .doc files. Repeat the same for .xls and .ppt files if required.
Cheers