Is there a way to tell who has accessed a file under Windows XP? (i.e. copied or opened but not modified a file)
No.
Real, secure Operating Systems (like MVS) have built-in security and access control subsystems & logs to track and maintain control over such things. Windows does not.
In Windows XP Professional you can set up auditing which will give you this information. You have to turn it on ahead of time though.