Telnet…

Assume you have sensitive files on your computer, and assume that it is connected to a network.
Now you might some shared folder to allow people on your network to access some files…
But what if I came and silently created an administrator account for me and enabled the telnet service on your computer…
Now I can use “dir” to navigate your computer, and “xcopy” to move some dorectories/files to the shared folder on your computer and then access the shared folder and transfer the files to MY computer!

Do lock your PC whenever you are not using it, you never know when I might be around!
Otherwise, use a guest account whenever you are in a public place and might need to leave your PC alone for few minutes. Leave the administration account to install programs and change settings.