The best way of proceeding is to use whatever arrangement your laptop has (restore disk, restore partition/utility or whatever) to restore it to factory condition; in the course of this, the system will be formatted. Then, before reinstalling Office, install Eraser and run a free space erase on the drive.
If, as with many laptops, there is a separate data partition (Drive D: or whatever), quick format this if you need to, then run a free space erase on that drive also.
If you can't find your Office install disk/licence number, sell the laptop without Office. It is virtually impossible to guarantee that you have removed all private data from a drive sufficiently well to let it out of your hands without both formatting it and erasing free space. This is not an Eraser issue; it's because files and copies of files will exist in all sorts of places you don't know about (pretty much however expert you are).
David