Microsoft Office software is included on a computer’s drive only if it was purchased in a bundle with the machine. Far more often, what is pre-installed on a hard drive is the Microsoft Office Test Drive version, a trial version that only works for 60 days or 25 uses. The problem with the Test Drive is that after your trial period expires, you won’t be able to save, edit or print the documents you created.
If you want to continue using Office, you can order an activation key (serial code) from Microsoft online to turn the test drive into the full product, you can install a retail version of MS Office, for non-commercial use you can get the Home and Student version of Microsoft Office which can be installed on up to three machines in the same household. If you are in university or college, check with your uni bookstore or computing services, they may have even better deals on the software.
It is usually recommended to do a de-installation of the Trial version of Office before installing a new Office product.
Don’t download pirated software — if you can’t afford Office, or prefer not to use Microsoft, then look at OpenOffice http://www.openoffice.org, NeoOffice http://www.neooffice.org/neojava/en/index.php or LibreOffice http://www.libreoffice.org/download/ – open source alternatives which will open and save most Microsoft Office format files.