Just been installing some software on the desktop running Windows 8, and noticed that my disc usage is really high considering what is actually installed.
Had a look through the folders, and found a Windows.old folder which was over 18GB in size. It appears what when you upgrade to Windows 8, it backs up the old Windows 7 installation so you can access your old files.

This is safe to remove according to the help site: http://windows.microsoft.com/en-GB/windows7/How-do-I-remove-the-Windows-old-folder
So, removing Previous Windows Installation and the Temp Install Files:

25GB reclaimed!