So if you have upgraded to Windows 8, then you may notice that you have a "Windows.old" folder in the root of your C Drive. If you try and delete it, you get an error of Access Denied. So here it how to delte it and recover the 30-50GB of space.
WARNING!!! Before using Disk Cleanup, make sure that all of your files and settings are in Windows 8 and are correct.
1. Open Disk Cleanup by swiping in from the right edge of the screen, tapping Search (or if you're using a mouse, pointing to the upper-right corner of the screen, moving the mouse pointer down, and then clicking Search), entering Disk Cleanup in the search box, tapping or clicking Settings, and then tapping or clicking Free up disk space by deleting unnecessary files.
If you're prompted to choose a drive, select the drive that you just installed Windows on, and then click OK.
2. In the Disk Cleanup dialog box, on the Disk Cleanup tab, click Clean up system files.
If you're again prompted to choose a drive, select the drive you just installed Windows on, and then click OK.
3. Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then click OK.
4. In the message that appears, click Delete files.