|You can't make the desktop computer a slave drive but as suggested you can make it into a server. You will need to link the two computers up in some manner. |
How, will depend on what hardware is currently in use. IMO the use of a router is a good method to protect your system. If you configure your hardware correctly a router will act as a hardware firewall.
Most routers are also switches. They have connection for at least 3 or 4 computers to be connected. Connecting your two computers in this manner will network them and also protect both from outside invasion, to a great degree.
Both computer would then be able to access each other and the internet, if so desired.