To my knowledge, no version of Windows has ever come with a free Microsoft Office included (though the computer manufacturer may have added a trial version of Office for you to use until it expired when you would need to purchase it if you wanted to keep using it). Also, you may have bought Office separately originally or a fully paid and functiional version may have been included with the computer though that is rare and they may have even installed it for you - again rare (but then you would have received reinstallation disks and an activation product key for that version of Office).
If you want Office now, you need to purchase it (or find a prior and separate installation disk along with the product key for it if you did actually buy it as a separate product - again, it has never been included for free with any Windows Operating System). If you are willing to pay for it and want "the real thing", then use Ashvin's link above. Many or most offer free trials (60 days I believe) so you can test it out before buying. I'd select the version with the most options for the trial so you can test them all and decide what you need (there may be some you haven't used before but find helpful) and then purchase the version that contains only what you require.
If you prefer not to pay but want something that offers almost the same options (similar programs but slightly different), then use the link provided by Richard. Keep in mind that some versions of Microsoft Office contain programs not included in OpenOffice (and OpenOffice does not include anything that is equivalent to Outlook which I cannot live without). Still, if all you need is something that does pretty much what is done by Word, Excel, PowerPoint, and Access (those are the only "substitutes" included in OpenOffice) and enjoy the option that they are totally free, then try it at the link provided by Richard. I recommend you at least give it a try (it's free) and see if it will work and thereby save yourself the expense.
I hope this helps.
Good luck!