Heh non-admin accounts for home PCs are a joke, half the games and too many apps won't work when you are not an admin! Maybe MS's next OS will have real kernel level permissions like linux? Or maybe linux will finally become a decent desktop OS?
File permissions are also a joke. Reinstall the OS or boot the hard drive in another PC and you can do anything!
What you need is encryption. Without the right key no one can decode the file, ever (well a supercomputer trying sequential keys will eventually generate the correct one, but not in your lifetime)
Windows XP has built in support for encrypted files and folders and the key is tied in with your account. No one will ever be able to open the file without loggin in to that account.
Anything bad happens to that account the files are gone forver - unless you export the key and keep it in a save place. I put the key into a password protected rar burnt 2 copies.
Everyone can still see the names of the files and folders though. And of course anyone who can log into your account can access the file.
There are also 3rd party tools (like PGP disk) that create a virtual encrypted drive letter that requires a password to enable. Not sure if they are any free ones.
I have also seen a cheap IDE passthrough device that scrambles/descrables data as it enters/leaves the hard drive. Without a hardware key the entire drive is gibberish.