You should have two Windows directories, one for each XP OS. Find the old Windows directory and look through it for things that you do not want to delete. It is possible that programs other than Windows have been installed into it, so make sure there is not anything there that you will need. It is a good idea to back your important files up before doing something like this. Then you should be able to delete the whole directory and all of its contents. When you do something this drastic, always expect major problems. If you don't want to risk it, don't do it. And be ready to reinstall Windows if this does not work.
A safer alternative to that would be to move the old Windows directory to a different drive. That way if there are problems, you can copy things back. Later, you can then delete it.