A post on WinMatrix.com refers us to a document released by the NSA titled “Best Practices for Keeping Your Home Network Secure” from April 2011. Among other things, they touch upon why Windows XP should be left behind in favor of more modern operating systems:

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 post on WinMatrix.com can be found here.
The NSA document can be found here (PDF).