VirtualBox has a built in RDP (Remote Desktop Protocol) Server which allows one to access the virtual machines remotely. Here is how to get this working (I am assuming you have VirtualBox installed and Virtual Machine created):
1. Download and install VirtualBox 4.0.8 Oracle VM VirtualBox Extension Pack .
2. Enable Remote Display setting by ticking off the enable server box for your virtual machine, if you want to access multiple machines you would have to specify different ports. Choose External for the authentication method, this allows you to use your Host OS user name and password for authentication (needs extension pack installed).
3. Use any RDP client such as Terminal Server Client (tsclient) available with most *nix distros, Windows users can try using mRmote  which is a decent RDP client and allows you to specify user name and password beforehand. The guest OS has to be running for this to work!
The username and password used for connection are of Host OS (physical machine) and not the guest OS (virtual machine)
Hit connect button and it should connect. Don’t forget to replace the Computer ip/host name with the one of your Host machine.