If formatting is not an option due to feared loss of data, you could use something akin to Partition Magic to resize the existing partition and move your valuables to a new partition temporarily, then format the system partition, install Windows XP, move documents back, and remove the extra partition.
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Nevermind, I think I misread what you were after. I'm not sure I understand what you mean by "it is not permitted." Are these W7 files located on their own partition? Or did you install XP on the same partition as W7, and it simply took over the partition and left the old Windows, Users, etc folders there, and won't permit you to remove them? Maybe you need to use Take Ownership on those folders, and change file permissions before you can remove them. Although it's been so long since I've used XP, I don't remember if it even understands Vista/7 permissions...
Oh how I miss the olden days when you could just use attrib to change file permissions and erase whatever you wanted, without Windows telling you what you can or cannot do with your own damn computer.