In case you have upgraded your computer from Windows 7 to Windows 8 or 8.1, during installation, the computer users files will be automatically copied to a folder named "Windows.old". This usually contains user created files, music and videos from your previous windows installation.
If you already verified that you don't need them anymore, you might want to remove this folder to get more free space, especially if you have a SSD drive. However, there is a bit of a work-around. Deleting the folder in Windows Explorer won't do it.
Instead, right-click on your C-drive and select properties. Under the "General" tab, click the 'Disk-Cleanup' button then click on the "Clean up system files"-button. Wait a few minutes then soon under "Files to delete" box you'll see a new entry called "Previous Windows installation(s)". Check that box and hit OK and Delete Files to free up some valuable space.