This tutorial shows how to enable remote control to virtual machine desktop.
Configuring virtual machine
First,open up VirtualBox and click on the virtual machine you want to connect to.Click Settings,select Display(or,Remote display) in left.Go to Remote Display tab and check the Enable Server.

virutal_remote_desktop

Here we need to remember the Server Port :3389 which will be used in connecting this desktop.

Connecting this virtual machine desktop
Connect from Ubuntu:
First,we need install this in the computer that we want to remote control to the virtual desktop.Run this in terminal(Applications->Accessories->Terminal)

sudo apt-get install rdesktop

Now,use this command to connect to the remote virtual machine:

rdesktop -u wraith -p password -g 1024x768 -a 16 192.168.1.100:3389

Here:
wraith is the username of the HOST.
password is the password for the user on the HOST.
192.168.1.100:3389 is the Host IPaddress and Server Port.

Connect from Windows:
Just go to Start->All Programs->Accessories->Communications->Remote Desktop Connection.And type in:
Computer: 192.168.1.100:3389
User name: wraith
Password: ******