Uninstalling Windows XP - Problems Downgrading to Previous Windows Version
Chances are low that by uninstalling Windows XP, you can get back your previous Windows installation as it was. There are several factors that create hurdles to get back to your previous Windows installation that would be working fine. Some of these hurdles are listed below:
1 The primary factor is the File System you chose while upgrading to Windows XP. If you chose NTFS, there is no way you can get back your previous Windows installation even after uninstalling Windows XP.
2 Windows Pagefile and several other system files should be removed manually in most cases. If left as such, they will interfere with the functioning of your Windows 9x OS.
3. Any software that you installed on Windows XP may not work with the previous installation. You will have to uninstall those programs as well - when you uninstall Windows XP.
Personal Note: I have always recommended people to switch completely to Windows XP and forget previous versions. Even if you need something say, Windows 98SE, you can have a dual boot for Windows 98 and Windows XP. The reason is that Windows XP is the best usable operating system from Microsoft after MS DOS. Most of the current software (Office 2007, Adobe CS (current ver CS3), Corel 8 (current ver 11), and many more) are not compatible with even Windows 2000, so I cannot figure out why people would opt for Windows9x versions!
Uninstall Windows XP and Revert to Windows 9x
If you had an upgrade to Windows XP from Windows 9x or Windows ME, you can uninstall Windows XP to get the previous Windows installation without much damage:
1 Press F8 while booting to get the Windows Boot menu.
2. Use Safe Mode to login.
3 Open Add/Remove Programs from Control Panel.
4. Click YES when prompted to uninstall Windows XP
IMPORTANT: You cannot uninstall Windows XP if you upgraded from Windows NT, Windows 2000, or XP Home. For these OS upgrades, you have to manually remove XP.
NOTE: If you cannot find Uninstall Windows XP in Add/Remove Programs, you will have to manually remove Windows XP.
Manually Uninstall Windows XP - Windows XP Not Present in Add/Remove Programs
IMPORTANT: The procedure to manually uninstall Windows XP may cause data loss. It is recommended that you backup all your data before you proceed. In case you cannot start Windows XP, use a 98 or ME bootable disk to access your data.
Once you are able to access your hard disk, look for folder named UNDO in the system partition, where you installed Windows XP. If you find the folder, look for the cab file named boot.cab. You will have to use the command line to uninstall Windows XP in this case. To open the command prompt, click Start, RUN, and type CMD in the RUN box.
In the Command Prompt Window, type in the following:
This command takes you to the root folder of the system partition. \ represents the root folder.
 EXTRACT c:\undo\boot.cab /e /y
This command extracts the files copied by Windows Installer into boot.cab while upgrading to Windows XP (for rollback).
 copy c:\$win_nt$.~bt\txtsetup.sif
The command copies the SIF file containing information about Windows upgrade to the root directory.
 copy c:\$win_nt$.~bt\bootini.bak
The command copies boot.ini containing information about “Windows 9x boot parameters” to the root directory.
 attrib -r -s -h boot.ini
Removes all attributes - hidden, read-only, and system attributes - from “boot.ini” so that it can be modified using following commands.
 ren boot.ini boot.xx
Renames current (Windows XP) boot configuration to a different filename
 copy bootini.bak boot.ini
Copies original Windows 9x boot file backup to “active boot file” so that you boot with your Windows 9x boot settings.
 Reboot the computer. On the boot menu, select Cancel Windows XP Setup and proceed to check if you previous OS needs any repairs. You may have to reinstall some programs once you are on your Windows 9x operating system.
Though a little bit tough, this is the best method to manually uninstall Windows XP from your computer.
NOTE: If you are not able to find the folder UNDO, it is recommended that you backup all your important data and perform a clean reinstall of your Windows 9x or Win Me instead of trying to uninstall Windows XP.