I have had a frustrating time trying to install Windows XP onto a PC that had previously had Vista installed on it (unactivated). I assumed that because the disk is recognised as NTFS and Windows can delete the existing installation, there would be no problem installing XP without reformatting the disk/partition.
However things are not that simple, and both when using a RIS image, and using a CD installation, I encountered various errors regarding files that could not be copied (which I skipped), then when the automated setup restarted in GUI mode, the setup was halted with a fatal Access Denied error.
Once I got wind of what might be happening and deleted the entire partitition, created a new partition and gave it a full format, RIS based XP setup proceeded normally without any further user input, the way it should normally work. I presume that the Vista setup had left ACLs and other settings on the disk that caused conflicts with XP Setup.