The only way you could "password protect" a file is to put it into a zip/RAR type archive, and add a password to it. But, that's not very useful if you want to use the files all of the time.
A better solution would be to look at implementing file permissions. That way, each user on the computer would have their own user names, and you can grant read/write/execute/modify type permissions on a per-user basis.
well, u can't really password protect a file, but u can encrypt it, so only a person on YOUR XP account can access it. What you do is right click the file/folder, then hit properties. Then under the "general" tab, click advanced, and then select ENCRYPT CONTENTS TO SECURE DATA and hit ok. when u hit ok on the next screen it'll give u the option to encrypt just the file/folder, or everything in the file/folder. i usually select EVERYTHING.
Even when u encrypt a file/folder, another user will be able to see the file/folder, but when the appropriate program associated with the file/folder tries opening it, the program will give an error message saying access denied.