If you want to do that, use a real multiuser OS. Windows NT (including XP) is Microsoft's excuse for a multiuser OS. All earlier versions, while some supported multiple "accounts", were not multiuser OSes. (Yes, a multiuser OS is an OS that supports activities from multiple users at once. See the Wikipedia article for a confirmation of that.)
The fact is that what you described would require special software that most people don't need. This would not work because Windows is designed for a single keyboard and mouse. It will get confused if you have multiple mice or keyboards connected -- connect two mice, use one in each hand, and watch your hands fight for control over the cursor.
If you want to use your system for multiple users at once, you can do that, but you will need at least one computer for each user. These "computers" don't need very much at all (they often don't even need an operating system, much less a hard disk) and are likewise called "dumb terminals".
asen, there is a reason there is such a thing as file locking. While it is (usually) purely advisory, file locks are useful to tell others "Hey, I'm writing to this file right now. Wait until I'm done (or reopen the file every few minutes) to get the latest version."