Here is the information about Windows Operating System support life cycle that you need to concentrate:

With the end of support (EOS) coming for several products, moving to Windows 7 and utilizing the many features built into the product can help you achieve a more secure and compliant platform for your users, reduce your help desk costs, and increase performance and reliability for your end users. Here are some key dates to remember:

March 1, 2010 – Windows 7 RC pre-expiration shut downs begin.
Beginning March 1, the Windows 7 Release Candidate (RC) will start to shut down every two hours in preparation for its expiration on June 1, 2010. That means you will need to migrate your RC systems to a released to manufacturing (RTM) version of Windows 7; or your previous operating system. Remember to back up all your data as either option will require a clean installation. For more information, read this post on the Windows Team blog.

April 13, 2010 – Support for Windows Vista RTM ends.
To help ensure your Windows Vista PCs stay secure and up to date, make sure they are running Windows Vista Service Pack 1 (SP1) or Service Pack 2 (SP2).

July 13, 2010 – Support for Windows XP SP2 and Windows 2000 ends.
If you are running Windows XP, stay more secure by moving to Windows XP Service Pack 3 (SP3) or migrating to Windows 7.

Updating your Windows 2000, Windows XP, and Windows Vista–based PCs before the end of mainstream support dates will ensure that your PCs stay supported and receive security updates. Migrating to Windows 7 provides the longest support lifecycle for your organization, helping to ensure protection, support, and timely updates.

For more information, visit our new End of Support center when it goes live later this month at www.microsoft.com/eos. Need more time to test your application and hardware compatibility in Windows 7? If so, the Windows 7 Enterprise 90-Day Trial provides a great opportunity for you to finish critical testing with a full version of Windows 7 Enterprise.