Windows 7 can be rolled back to the previous operating system only if you have used the Windows 7 installation media to install Windows 7 to the same hard disk drive on which you had Windows XP, Windows Vista, or another version of Windows 7 installed. This applies to situations when you upgraded to Windows 7 using both the Upgrade media and the Full product media. In such kind of installation the Windows 7 installation would have created a Windows.old folder that contains your previous operating system and personal files. The Windows.old folder is in the root of the Windows partition. To revert to this previous operating system first find out if your computer has the required disk space available or not, and then follow the steps given in this article.
How to find if required space is available?

1. Go to Start ->Computer.

2. Click Details on the View menu.

3. Check the Free Space column and note how much space is available for Local Disk (C:) under the Hard Disk Drives area.

4. After noting down the free space size open Local Disk (C:) and then determine whether the Windows.old folder exists.

Please not if the Windows.old folder does not exist you cannot follow the steps in this article to restore the previous Windows installation to this computer. You must backup your files and install the previos operating system as fresh and then restore the files again.

5. If the window.old folder exists Right-click the Windows.old folder. Operating system will determine the size of the folder.

6. Check if the size of Windows.old folder is smaller than the free space available on Local Disk (C:). If the Windows.old folder is two times as large as the free space available on the C drive then you may not be able to restore the previous Windows installation.

Steps for Rolling back to previous Operating system

Step One: Start the Windows Recovery Environment and then inserts the Windows 7 installation media in the DVD drive. After this please Restart your computer. Press any of the keys when you are prompted to press for booting from the disc. Select a language, time, currency, keyboard input method in the “Install Windows” window and then click on Next.  Click on “Repair your computer” In the “Install Windows” window. In the “System Recovery Options” window click on the version of the Windows7 operating system that you want to repair and then click on Next. Click on Command Prompt In the “System Recovery Options” screen. The Command Prompt window opens and it displays the prompt. The command prompt window is the place where you will type the commands that are given in the following steps.

Step Two: Move the Windows 7 folders to a new Win7 folder Type the following commands and press ENTER after each command: C:  Md Win7  Move Windows Win7\Windows Move “Program Files” “Win7\Program Files”  Move Users Win7\Users  Attrib –h –s –r ProgramData  Move ProgramData Win7\ProgramData  Rd “Documents and Settings”

Step Three: Copy the contents or move the contents of the Windows.old folder. Type the following commands and press ENTER after each command: move /y c:\Windows.old\Windows c:\  move /y “c:\Windows.old\Program Files” c:\  move /y c:\Windows.old\ProgramData c:\  move /y c:\Windows.old\Users c:\  move /y “c:\Windows.old\Documents and Settings” c:\
Step Four: Restore the boot sector for the previous Windows installation Type one of the following commands at the command prompt, as applicable.
Type the following command, and then press ENTER: D:\boot\bootsect /nt52 c: When the previous Windows installation was Windows Vista
Type the following command, and then press ENTER: D:\boot\bootsect /nt60 c:
Step Five: Restore the Boot.ini file for the previous Windows installation of Windows XP or of Windows 2000 Type the following commands and press ENTER after each command: Attrib –h –s –r boot.ini.saved  Copy boot.ini.saved boot.ini
Step Six: Close the Command Prompt window, and then click Restart Type the following command at the command prompt, and then press ENTER: exit Click Restart to restart your computer.