If NexGen is so advanced how come it can't support Windows Vista or Win 7?

I found this on FAA's website today 12-01-09. 

The FAA CIO and the Vice President for Acquisition and Business Services require that all Lines of Business (LOBs) and Staff Offices (SOs) to refrain from acquiring Microsoft's "Vista" 

operating system, Office 2007, and Internet Explorer 7. These three products are currently not supported by 
NexGen. The current FAA standards are the Microsoft XP operating system, Office 2003, and Internet Explorer 6.

I think the agency is finally getting some Pentium computers this year. Bye-bye 486. 

NexGen: The future is yesterday