Do you see a Windows.OLD folder in the root of the partition, just a single Windows folder, or two different Windows folders? Usually Vista will replace the XP installation and move the relevant files into the 'backup folder.' In that case there is no way of fixing it from within Windows, and since Microsoft doesn't support the downgrade of Windows there's no simple way to revert back to XP outside of Windows either. At that point you're looking at a patchwork operation, manually restoring the appropriate files while booting to a Linux Live CD or connecting your hard drive to another computer as a slave. In short, this is far from an easy task. Is there a specific reason you need to revert?
John
hi, i inslalled windows vista in the same partition of my old windows xp, how can i delete the vista folders (windows and program files, etc.) from the vista installation, windows doesn't letme. acces denied...

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