I installed virtualbox as I needed a quick way to use iTunes so I didn't have to keep shutting down my laptop to use it and then restart it again to go back to linux. I think the installation failed though and I can't really be bothered to try and redo it, so I uninstalled it (from the software center) I just want to know if I can get the storage I allocated to windows (9.5 GB in this case) back? and if not, how do I get it back?
With VirtualBox, you allocate yourself a Virtual Hard Disk as you mentioned above.
You mentioned you allocated 9.5GB to Windows. Simply delete this Virtual Drive and all of that space is given back to you.
As the answer from user146001 mentions, in your Home directory, you enable "Show Hidden Files" (or press Ctrl + H to see hidden files) and find your .VirtualBox folder. In there, go to your HardDisks folder and simply delete the .vdi files, as these are the files of your virtual drives.
You can go about this via the terminal or via the file manager.
I haven't deleted Vbox for awhile. However what I found was that in your "Home" directory, then under that there is a directory called ".VirtualBox". If you don't see it enable "Show Hidden Files" in your View. Under the .VirtualBox directory is a "HardDisks directory. Go in there and delete the ????.vdi files. That is the allocated space for your Windows. You should get the space back. Best of Luck.