If you are interested in downgrading Windows 7< to XP, you can read the following post. Presently, there is a similar solution as posted in the earlier post, on downgrading Windows 7 to Vista. The commands are common, while there may be small variations.
Pre Requisite :
Windows.old folder should exist in the root of Windows 7 drive, with previous contents while upgrading from Vista to Windows 7.
I will detail the steps necessary in simple manner, while detailed screenshots and steps can be got from the source.
1. Sufficient free space must be available on the Windows 7 drive. This space should accommodate transferring Windows 7 contents to a folder and copy Vista contents from Windows.old folder.
2. Note the Windows 7 OS drive letter and DVD drive letter.
3. Insert the Windows 7 DVD installation disc and boot from the disc.
4. Select “Repair your Computer” and then your Windows 7 OS and click on Command Prompt.
5. Now type the following commands in the command windows and type Enter after each command.
C: ( Your Windows 7 drive letter)
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”
6. Now copy the contents of Windows.old folder to the drive.
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:\
Here, C: is the Windows 7 drive letter.
7. Restore the boot-sector of your Vista OS by typing the following commands.
D:\boot\bootsect /nt60 C:
Here D: is the DVD drive letter and C: is the Windows 7 or root drive.
8. Exit and re-boot your computer. You can also delete the Win7 folder, to gain some disk space, present in the root of your Vista OS.