Hey :)
when you are trying to copy files from c: to the new installed os on d: I am going to guess that you are trying to get the My Documents and such, is this right?
if so read on :)
Windows ME, XP, Vista will protect these files by ownership rights so if we pretend that the user account on c: was john and the account on d: is John Smith you dont have the rights to view these files so what you need to do is
On XP Home start in safe mode go to the documents and settings folder on the old c: RIGHT CLICK on the Documents and settings and select properties, now select security tab and there should be a button called take ownership is XP Pro you can do this in normal mode you dont need to boot into safe mode,
you may need to select in the security tab ADVANCED to get to the take ownership button Im srooy I can not remeber but a quick google search should take you to the MS webpage that explains it in fuill detail,
hope this helps