Network computer login and permissions

> Hello, I am a fairly experienced windows user and am in charge of
> maintaining
> a home network of a little more than half a dozen computers. Being the
> administrator, I'd like to be able to share large portions of a person's
> harddrive and allow me to connect to it remotely.
>
> I'm having a problem with logging into a person's computer. Some of them I
> have already logged into with their user name/passwords and thus have
> limited
> access to the computer. I can find no method of logging in with different
> credentials, so how is this done?
>
> I'd like to set up the permissions for the shared folders so that my
> current
> user would be able to access all the computers in the network. But in the
> folder permissions dialog, I cannot add any user that is not on that PC
> locally. Is it possible to add a "location" to the dialog so I can do
> this?
> Or any other method?
>