small xp networks: allow/deny user access to pcs

I have 12 xph pcs interconnected at a 16-port switch. Internet connection is also made at the switch.Some pcs each have a single dedicated cable from 16 port switch. Other pcs have one cable to a room, then split out using a switch. Everything works fine, except this: I (admin)want to have access to c:\, or root, of every pc, from any pc, but I don’t want anyone else to be able to do it. In short, I want password-protected entry to root, like in win98. Third-party software may be acceptable. Changing os is not.
Thanks for the help….