Uninstalling Windows Vista and restoring the previously deployed version of Windows – be it Windows XP, Windows Server 2003, Windows 2000 or even a low level edition of Vista – is quite a task. If you don't know your way around the command prompts, then you'd better stay away from this one or ask for help. But otherwise, there is nothing stopping you from manually removing Windows Vista and replace it with the previous Windows platform you had installed. But be advised, the process does require a "steady hand" as it is laborious and full of meanders. Microsoft has yet to automate the removal of Windows Vista and there are
no signs that point to this feature with the first service pack for the operating system.
The only prerequisite for the uninstallation of Vista to work is the existence of the Windows.OLD folder on the partition with the operating system. This folder contains all the files from the previous installation of Windows, and without it, you cannot complete the task of removing Vista and switching back to the Windows version you were using before. If you deployed Vista on C: , then here is where you can find Windows.OLD. You will also have to make sure that the folder's size is not larger than that of the free space available on the partition.
Now via the Windows Vista installation disc, start the Windows Recovery Environment. Choose the option "Repair your computer" after you boot from the Vista DVD, and in the "System Recovery Options" select the Vista version you want to uninstall and click on "Command Prompt." A little renaming is in order for some Vista folders, make sure to enter the following commands in the same order as presented in here and press Enter after each one: "c:"; "ren Windows Windows.Vista"; "ren "Program Files" "Program Files.Vista""; "ren "Users" "Users.Vista"" and "ren "Documents and Settings" "Documents and Settings.Vista"".
Next, you have to deal with the contents of the Windows.OLD folder. "move /y c:windows.oldwindows c:" and "move /y "c:windows.oldprogram files" c: "" are the first commands you have to enter. Then, if you want to restore a previous installation of Vista type "move /y c:windows.oldusers c:". For Windows Server 2003, Windows XP, or Windows 2000 enter "move /y "c:windows.olddocuments and settings" c: "". And of course hit Enter after every line.
Having handled the contents of the Windows.OLD folder, you will have to bring back to life the boot sector from the older Windows operating system. Keeping in mind that DVDdrive is the letter for the DVD device on your system, for example E:; for Windows Server 2003, Windows XP, or Windows 2000 enter "DVDdrive:bootbootsect /nt52 c:"; and for Vista "DVDdrive:bootbootsect /nt60 c:".
Right, the boot sector is in place, so it’s time for the Boot.ini file. If you are switching back to Windows XP or to Windows 2000 then follow the next steps "c:"; "attrib boot.ini.saved -s -h -r"; "ren"boot.ini.saved""boot.ini"" and "attrib boot.ini +s +h +r". Press Enter after each item, you don't need to do anything for Vista or Windows Server 2003.
Now all you have to do is type "exit", hit Enter, and then restart your machine. Windows Vista will be gone and you will boot into your old version of Windows.
Source:→ softpediaMicrosoft, Windows Vista, Uninstallation, Restoration, Windows XP, Windows 2000, Windows Server 2003, Tips and Tricks