Could you explain the current situation better, please?
You claim that you have separate logins, but no security. That would imply that you need to prevent them from logging in. The obvious solution is to lock your screen when you leave rather than logging out, but that's so obvious that there must be a reason you can't do that.
You also say "allow the pc to get into windows", implying that maybe it's being turned off, as the only time the PC "gets into Windows" is when it boots. If that's the case, check your PC's BIOS to see if allows you to set a BIOS password. Many modern PCs do. If so, use that.
There are systems that allow you to connect and disconnect the hard drive with a key, but somehow I doubt reconfiguring hardware is within the realm of things that you're allowed to do.
You could look at a fingerprint reader. I'm sure that the software that comes with them will allow you to configure it so that a fingerprint is required for login.
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Bitt Faulk