The Microsoft Application Compatibility Toolkit (ACT) version 5.5 contains the necessary tools and documentation to evaluate and mitigate application compatibility issues before deploying Windows 7, Windows Vista, a Windows Update, or a new version of Windows Internet Explorer in your environment.

ACT helps customers lower their costs for application compatibility testing, prioritize their applications, and deploy Windows more quickly.

You can use the ACT features to:

• Verify an application's compatibility with a new version of the Windows operating system, or a Windows Update, including determining your risk assessment.
• Become involved in the ACT Community, including sharing your risk assessment with other ACT users.
• Test your Web applications and Web sites for compatibility with new releases and security updates to the Windows® Internet Explorer® Internet browser.

You can get the tool from here.

Further information on the Windows Application Compatibility page on TechNet.