Nice article from MS about protecting and managing unmanaged clients..
Security organizations and professionals now understand that internal network risks, whether intentional or accidental, have the potential to be even more perilous than external threats. Many organizations have made significant investments of time and resources in areas such as patch management, anti-malware solutions, and identity management initiatives. To maximize the effectiveness of their investments and to ensure that these investments are used universally, organizations must find ways to efficiently enforce security policies. Rogue computers can be a problem for system administrators and security professionals. Such noncompliant computers pose a number of risks, from being vulnerable to malware infection to being potential platforms for an attack. Traditionally, they have been difficult to manage and bring into compliance. This security guidance discusses some effective approaches that can be used to help enforce compliance with security policies. These approaches maximize the benefits of risk management efforts and add an extra layer of security to midsize business networks that will help reduce the risks associated with untrusted and unmanaged computers.