1. Buy a new Vista Capable PC with Windows XP this holiday season from Dell, HP or other hardware vendors and you'll get a coupon for a free upgrade to Windows Vista.
2. Attend the 2007 International CES conference in Las Vegas as an exhibitor. Microsoft will provide free Windows Vista copies to all exhibitors at the annual Consumer Electronics Show to run on computers they are using in their booths at the show.
3. Nominate yourself or ask a friend to nominate you for the Microsoft MVP program - they will consider the request only if you have made some decent contributions to the tech community (not necessarily Microsoft) in the last few months. As an MVP, you get loads of Microsoft software including free licenses of Microsoft Office 2007 Ultimate and Windows Vista Premium.
4. If you have participated in the Windows Vista Beta testing program and have submitted at least 1 bug report, Microsoft will send you a complimentary copy of Windows Vista Business or Ultimate. Check your inbox for the Vista Download instructions from Microsoft.
5. Register for the PowerTogether webinars and Microsoft will ship you a fully licensed version of Windows Vista for free.
And if none of these options work for you, download the Windows Vista Transformation Pack 6 here and make your Windows XP (or 2003) look like Vista. The program will replace you old XP boot screen, visual styles, toolbar icons, etc to resemble Windows Vista aero-styles.
Vista Transformation Pack is like a new Windows XP Theme that will replace a few system files on your XP PC, so it's always nice to create a system restore point before installing the software.
Related: Free High Resolution Vista Wallpapers