It is not every day that we see a Government agency telling us what platform we should use for our business and personal computing needs. While it is known that Vista and Windows 7 provide better security enhancements than Windows XP, the NSA is now calling it a best practice to upgrade from XP to a newer OS.
The NSA (via winbeta.org) is not typically known for providing software recommendations to the general public. It is also a bit curious for the NSA (who is known for its domestic spying) to offer up ways to better protect yourself considering the entire organization is based around information gathering. Besides the conspiracy theories around this document, it does have value. The NSA states:
Both Windows 7 and Vista provide substantial security enhancements over earlier Windows workstation operating systems such as XP. Many of these security features are enabled by default and help prevent many common attack vectors. In addition, implementing the 64-bit mode of the OS on a 64-bit hardware platform substantially increases the effort of an adversary to attain a system or root compromise. For any Windows-based OS, verify that Windows Update is configured to provide updates automatically.
The document also talks about OS X and that users should keep their OS updated, but falls short of making a specific version recommendation.
If you can overcome the idea of the NSA telling you how to best protect your data and identity, then the pdf (link) provides a resource that can be utilized by many, from the basic user to advanced IT professionals. At the end of the day, it is about informing the end user who needs to understand that security is a revolving policy, what worked five years ago, may not be sufficient today.