This KB971760 article describes how to manually restore a previous Windows installation on the computer to replace the current Windows 7 installation.

To do this, you must use the command prompt, and you must type specific commands at the command prompt to rename and to move folders between the different versions of Windows.

If you are planning to this, this article will be of big help you you!

How to restore a Windows 7-based computer to a previous Windows installation by using the Windows.old folder