I did a mistake of installing Windows Vista to a 10gig partition under VirtualBox.

It was enough to install but it was constantly complaining about lack of space.

To overcome this problem I resized the VDI then set Windows to use all the space.

To do this you can follow this process

1. Make sure you are running the latest VirtualBox. I am not sure what version the functionality came in but I successfully used version 3.1
2. Create a new VDI file the size that you want the new drive using the VirtualBox tools
3. Navigate to the folder containing your VDI files. On Ubuntu this is usually

~/.VirtualBox/HardDisks

4. Run the following command  replacing the file names

VBoxManage clonehd --existing oldhd.vdi newhd.vdi

This command will take awhile to run.

5. Once it is finished update your Virtual Machine configuration to use the new VDI then start the Virtual Machine
6. In Windows click your start menu
6.1 Right click on "Computer"
6.2 Select "Manage"
6.3 Select "Disk Management"
6.4 Right click on your main drive it should be Disk 0 if you only have one VDI
6.5 Select "Extend Volume" 
6.6 Select "Next" and you are done!