Windows and OpenOffice are two different things.
All (modern) computers need an operating system just to run. An operating system allows users to work with the machine's hardware, and also to provide a framework for applications to operate. Windows is the most popular operating system around today. Others include MacOS and the various flavours of Linux, e.g. Ubuntu, SuSE etc.
OpenOffice, however, is application software. It is a suite of programmes which include word processing, spreadsheets, database, presentation graphics etc. It is similar to Microsoft Office, but free.