For a (in my opinion) small price microsoft will offer you Windows Vista Ultimate for free, you simply have to let microsoft monitor your every move for 3 months, fill in 2 or 3 surveys and walla !, Vista is yours. Its limited to 18+ and USA residents only at the moment. You must be using XP or Vista (although this would make it pointless, would it not ?)
N4G.com : Microsoft Offers Free Copy of Windows Vista Ultimate if you Surrender Privacy
N4G.com : Microsoft Offers Free Copy of Windows Vista Ultimate if you Surrender Privacy