Windows XP Professional x64 Edition Preview
 

A really big advantage of the new Windows XP Professional x64 Edition OS is its ability to allow smooth transition from 32-bit applications to 64-bit ones, because it supports both: usual 32-bit software and the new 64-bit programs. Moreover, our today’s test session showed that 32-bit programs are performed in Windows XP Professional x64 Edition without any significant performance losses. This way, if you are a happy owner of a CPU with AMD64 or EM64T support, you have green light to install Windows XP Professional x64 Edition. Just keep in mind that it still might be incompatible with some specific software and hardware, so you’d better double-check first.

http://www.xbitlabs.com/articles/cpu...86-64-rc1.html