Enterprises w/ 10k+ seats, OEMs, MVPs and governments can get access to Windows source these days. Microsoft launched the program in 2006 or so to dampen the "Linux is more secure because we can see the source!!" FUD.
Government Security Program: Addressing the unique security requirements of governments worldwide by helping government actively participate in ensuring the security of their critical systems. We help enhance system security by providing access to Microsoft Windows and Office source code, prescriptive and authoritative security guidance, technical training, security information, and Microsoft security experts.
The Common Criteria Evaluation and Validation Scheme (CCEVS) is a form of software accreditation that focuses on software security.
You can view the results for the Windows 7 accreditation at [1]. The website also has comprehensive documentation on the methodology used to accredit the software (including visibility of the source code).
