If you find command prompt in the start menu, right click on it, choose properties and then run as administrator. Type:
cleanmgr/sageset:1
Press enter.
You will see a box appear which is basically a more advanced version of the Disk Clean app.
Scroll through the checkboxes until you see either 'old windows installations' or 'windows.old' and check the box. Do not check any box that your unsure about so it's probably best you stick just to the Windows.old checkbox. Once you've checked the box click ok and the box will disappear.
All you need to do now is to run the set up you've just created. Once again open an admin command prompt. Type:
cleanmgr/sagerun:1 (notice how this command differs from the above)
Press enter...