Installing Pre Vista windows after installing Vista

1. To install a previous version of Windows onto a computer running Windows Vista (Post-Beta 2)
With Windows Vista Beta 2 as well as in Windows Vista Build 5456, the only command one needs to run is:
bootsect /NT60 ALL
(Copy bootsect.exe from the boot folder of your Windows Vista DVD and paste it into your Windows XP Windows\system32 folder)
This pulls all Windows operating systems into the Windows Vista boot menu.

Login Logoff looping while starting windows XP

http://support.microsoft.com/kb/555648


Edit these values and type the correct path of shell : Shell = explorer.exe Userinit=X:\windows\system32\userinit.exe NOTE: These files may also be deleted by spywares. You may need to extract them using Windows CD. Steps for rectifying this problem:
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Log on to a networked computer.
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Run Regedit.exe
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Point your cursor to HKEY_LOCAL_MACHINE
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Select File > Connect Remote Registry
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Type computer name (infected computer)
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Navigate to the following location in registry of destination or infected computer HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon
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Edit these two values in right pane: Shell Userinit
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Change these two values to Shell=explorer.exe Userinit = x:\windows\system32\userinit.exe
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Exit from Registry
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Restart Infected computer.
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You should be able to log on to computer.