http://microsoft.productorder.com/clientx64/default.aspx
he Technology Advancement Program allows customers who purchased a 32-bit version of Windows with x64 hardware to exchange it for the corresponding x64 version. The Technology Advancement Program will be available from April 25 to July 30,
Customers must have purchased or built their x64-based PCs between March 31, 2003 and June 30, 2005. The program ends on July 31, 2005.
EDIT
Customers with versions of the product other than Windows XP Professional can either upgrade to Windows XP Professional and then make the exchange or purchase the Windows XP Professional x64 Edition CD from a System Builder, Distributor, or pre-installed on a new PC.