To make it easier for households with multiple PCs to make the move to Windows Vista, Microsoft is launching a limited-time offer for customers who buy retail copies of Windows Vista Ultimate. From Jan. 30 through June 30, the Windows Vista Family Discount will allow North American customers to license two additional copies of Windows Vista Home Premium for use on other PCs in the home at the reduced price of $49.99 each.
http://www.microsoft.com/presspass/press/2007/jan07/01-17ConsumerOptionsPR.mspx
If you plan on being legal about it, here's a good way to upgrade the PC's in your house with Vista...