if you are the main administrator type that uses the computer, just change everyone else to a "power user" and give them alot of the administrator type accesses. Then go and make sure thier "my documents" does not point to your "my documents".

(right click on the "My Documents" icon on your desktop or start menu and make sure the Target is something other than your My Documents Folder.

ex.
aaa logs in = C:\Documents and Settings\aaa\My Documents
bbb logs in = C:\Documents and Settings\bbb\My Documents


Also are these files you see from the other account in the "shared folder" if so, just turn off the file sharing for both accounts to eliminate this problem.

Sorry i can't type anymore atm, but i got work in 30mins =0... i'll prolly post some more from work

--Cyberfire

-Ninja-edit
make sure you also check the "Make this folder private" checkbox under the Sharing tab of your my documents folder, and any other folder you want to keep private ^_^.