What I do to make my VirtualBox Windows XP install accessible from anywhere is the following.
I open a terminal window and open a new session with SCREEN.
I start my VirtualBox Windows XP machine using
VBoxHeadless --startvm "WindowsXP".
I detach from my screen session.
After a minute or two (the time it takes for Windows XP guest to load) I can RDP into Windows itself.
I have also installed www.logmein.com on the Windows XP guest so that I can access it from over the Internet.
I am using the Open Source Edition of VirtualBox on FreeBSD Host so I don't have the built-in RDP functionality mentioned in the comments elsewhere.
Also here is a good article that goes through steps for working with VirtualBox from the commandline.