You are correct that Windows XP x64 is not supported. I would just like to clarify why this is. XP x64 was released in close proximity to Vista x64. Because of this, the decision was made to develop our drivers and software to support Windows XP in a 32 bit capacity and Vista in both a 32 bit and 64 bit capacity. As such, we have many drivers that are either compatible or are in development for Windows Vista x64. These online articles cover our plans for expansion into the 64 bit realm.
Get Ready for Windows Vista
A Closer Look at Windows Vista, Part III: 32-Bit vs. 64-Bit Windows
There are currently no plans to introduce support for Windows XP x64. However, some of our customers have independently developed some applications that work successfully on Windows XP x64. Hopefully, some of the community members will be able to detail the successes they have experienced. Good luck!
Seth B.
Principal Test Engineer | National Instruments
Certified LabVIEW Architect
Certified TestStand Architect