I don't think I can specifically advocate using pirated software, but It seems it may be easiest for you to find an alternative to buying a CD and CD Key(borrow it from a friend, download it and use someone else's CD key, etc.).
Seems that Open Office may be perfectly fine for personal word processing, but from what lindy says about his/her experience with Microsoft Office's compatibility, It seems the formatting of your documents could change drastically.
I started using Open Office after you posted this so that I could have some experience with it. I would recommend it strongly for something that will be accessed only from your computer. I would not recommend it to be distributed through e-mail to someone who may be using Microsoft Office.