How to remove the Windows.old folder ?
If you're going through the process of reinstalling the OS, it is usually wise to delete previous partitions and reformat the HD to clean it of any old files and possible malware.
However, if you install Windows by performing a custom installation and don't format the partition during the installation process, files that were used in your previous version of Windows are stored in the Windows.old folder.
After you're confident that your files and setings are back where you want them to be, you can safely reclaim disk space by using Disk Cleanup to delete the Windows.old folder. Note this can not be undone.
1. Open Disk Cleanup: right-click on your disk drive in Windows Explorer and choos Properties > Disk Cleanup.
2. Choose Clean up system files
3. Select the Previous Windows installation(s) check box, and any other check boxes you may want and click OK