After installing the Windows 10 Technical Preview I noticed my hard drive was running a little low on space. After a quick glance I notice the old windows.old file on my C: drive. Basically this is just your old version of Windows which I believe is kept around in case you need to restore the previous version of Windows (in my case 8.1).
This can be safely deleted, but if you try to do it manually you’ll find you need to grant rights and really fiddle around with things to get it completely removed. The best option is to use Microsoft guide for Windows 8 to delete this and other non essential stuff (logs, old drivers etc…).
The process is pretty simple.
In the Disk Cleanup dialog, tap or click Clean up system files. If you’re again prompted to choose a drive, select the drive you just installed Windows on, and then tap or click OK.
Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then tap or click OK.
In the message that appears, tap or click Delete files.
In my case this freed up about 30gb’s of space which on my 128gb SSD was pretty fantastic!