File sharing with Windows XP Pro

I’m working on a small network with 10 computers all running xp pro. Every system has mapped drives to evry other computer and all share the root C drive. The problem is when one person opens a file or document and then alters it and saves it, another person at another station who wants to copy and paste that document to their own system can’t. Acess denied. You have to remove the share and recreate it to get the file. How can I stop this from happening?