VNC is what I use on my home network. My server/tower lives in a closet without monitor, keyboard, or mouse, and when I need to access it to tweak the software (or queue up new requests on the file sharing network clients ...

) I run the VNC client on my laptop.
As noted VNC totally takes over the machine. It is like having a second keyboard and mouse attached to the computer. You can launch and close the server on the host computer at any time though, so you can control when your assistant would have access to your system.
However, VNC is
not particularly efficient at file transfer. I can kick
both computers' (laptop and server) cooling fans into high gear if both systems are already moderately loaded and I try and cut or copy files from a remote window and paste them into a local one.
If file access is all you need, and you don't really need remote
control of your computer, have you considered just setting up a secured FTP server? Old fashioned, but it would do the trick, and you can find freeware that would do it.
Even if control is something you need, using VNC for remote system adjustment, and firing the data through an FTP server works - it is how I get large files from my server to my laptop (or I use a flash drive, but that's too easy).