“You cannot uninstall Windows Vista by using the "Uninstall a program" option of the Programs and Features Control Panel tool,” informs Microsoft's Knowledge Base article 925809. In case you have upgraded to Windows Vista from Windows XP, Vista will not present an Uninstall option in the Programs and Features Control Panel tool. However, there is a way to revert to Windows XP, or to the previous version of Windows.
But first off, make sure that when you upgrade to Windows Vista you do it in such a manner to save the previous copy of the operating system in the Windows.old folder. In the absence of the Windows.old folder you will not be able to uninstall Windows Vista. And just to be safe, backup any essential data on your machine before proceeding with the uninstall process.
Boot from the Windows Vista DVD, but choose to repair your computer when prompted. Select the copy of Vista you wish to uninstall and click Next and then launch command prompt. Type “c:” and press enter, and then “cd” and enter. Then type “ren Windows Windows.Vista” press enter and “ren “Program Files” “Program Files.Vista”” and press enter again.
Then write the following commands and press enter after each one “rd “Documents and Settings”,” “xcopy /s /h c:windows.old*.* c:” and make sure you agree to overwrite the files by pressing y.
Now in order to restore the old boot sector you will have to type “D:bootbootsect /nt52 c:” and in case of an error just select another letter, something like this “E:bootbootsect /nt52 c:”. Now type “c:” hot enter, then “cd” and enter, and then “attrib boot.ini.saved -s -h –r” and “ren “boot.ini.saved” “boot.ini”” each followed by an enter. The final command is “attrib boot.ini +s +h +r”, press enter and type exit, press enter and click restart in the System Recoveri Options window.
Your previous copy of Windows should be in place. Just delete all the files that Windows Vista has left behind.