When you install OpenOffice, you are asked to specify the default program that ppt, doc, and xls files will open with. Perhaps you thought that you were just trying OpenOffice and didn’t select it as a default.
You can change the default program using the following steps (in Windows).
In Windows Explorer, locate a file that you want to associate with OpenOffice (eg. a file that ends with extension xls, like the example below).
Right mouse click on the file and show the context menu.
Select ‘Open With’ > ‘Choose Program…’
You may get a window like the following. Choose ‘select the program from a list’
You can then select the OpenOffice program and then tick the box underneath that says ‘Always use the selected program to open this type of file’.
Note: You will need to do this for each of ppt, doc, and xls files (and docx, pptx and xlsx files as well).