On the Windows Developer Preview downloads page it says
Notes about installing the Windows Developer Preview
You can't uninstall the Windows Developer Preview. To reinstall your previous
operating system, you must have restore or installation media.
Refer similar questions
Should I install the Windows 8 Developer Preview on top of Windows 7?
How to revert to Windows 7 after installing Windows 8?
WINDOWS.OLD folder contains all your Windows 7 files, it may contain data which you may want to keep, so back it up to an external drive.
Using this article from Microsoft support you can recover data from
windows.old file. The summary for the article says
This article describes how to manually restore a previous Windows
installation on the computer to replace the current Windows Vista
installation. This article is intended for a beginning to intermediate
The article is intended for Windows Vista, but you can follow the steps and see if it still applies for reverting from Windows 8 Developer Preview to Windows 7 and then again try installing(dual booting) Windows 8 Developer Preview once you have successfully reverted back to Windows 7.
For dual boot refer these
Does Windows 8 Developer Preview 64bit allow multiboot when installing from a mounted ISO?