I don't believe there is any real difference between the OS's in this regard. More than likely, the diffrence is either in your setup, or in which versions of the app you're using.
If you're using a newer version eMule, it may be doing something different with the file creation. Or, perhaps you're setup is different with your temporary files on a different partition than your "complete" folder in xp. This would require eMule to create a new file on the "complete" folder (deleting the old temp file) when the download was finished, while when on the same partition it may simply rename the temporary file to a new location, keeping it's original creation date.
The fact is, both XP and 7 treat this the same way. When the file is created, is it's creation date. period.