if its windows XP or above and u can setup the DDNS and port-forwarding for it, than Remote Desktop Protocol is a good solution as it seamlessly integrated into windows and for me, the best performer of the other solution.
But the hassle of setting it up is a down side, for rapid deployment Teamviewer or LogMeIn is a better solution especially Teamviewer. It helps with ppl behind firewall and NAT networks. Both of them use SSL connection.
Try Teamviewer. Leaving a computer on like this with an open way for people to access it is dangerous though.
I believe you are talking about VNC which by default, is not a secure protocol. Password is transmitted in plain text and connection are not secured. While some of its derivatives do have addons or plugins that encrypt the connection, one can also tunnel using SSH or VPN for extra protection. I wouldn't recommend VNC for Windows to windows remote desktop operation because like RDP, you need to config your router and firewall. Plus, the lessen security factor. But it is great for crossplatform remote desktop