I don't know about Office 2000, but I do know that Office 2003 works with Windows 7 - my mother was using 2003 on a Windows 7 machine for several years without problems, until she upgraded this year. 2003 also predates the "ribbon interface", so it should hopefully provide an interface not to different from what you'd be used to.

OpenOffice, the last time I used it, was about ninety to ninety-five percent compatible with Microsoft Office; I suspect they've gotten closer in compatibility since then, and my troubles were with Excel, not Word.