Define "copying".
By default, the Windows version of iTunes behaves differently than the Mac version. On a Mac, when you drag-drop files into the iTunes window, iTunes copies -- makes a duplicate -- the file into the iTunes folder stored inside the user's home folder. If you then delete the file you just dragged into iTunes, the copy that iTunes made still remains and is playable. On Windows, iTunes does not make a duplicate; it only adds the reference to the file to the iTunes music list. If you delete the file you just dragged into iTunes, iTunes will report the song as being not found. Both of these default behaviors can be changed by the user.
So when you say "copied" are you saying "cannot be added to iTunes" (the default Windows behavior) or are you saying "iTunes is unable to make a duplicate" (the optional Windows behavior)?
Where did these MP3 files come from? Do you have ANY music in iTunes? If so, can you export one, delete it from iTunes, then re-import it?