You can't uninstall the Windows Developer Preview. To reinstall your previous operating system, you must have restore or installation media.
Windows Developer Preview with developer tools
Windows 7 and Windows Vista Clean install only
Windows XP Clean install only
Windows Developer Preview (32-bit or 64-bit)
Windows 7 and Windows Vista You can keep accounts, files, and settings
Windows XP You can only keep accounts and files
A clean install is supported on all builds. You will only receive the full set of installation options when setup is launched in Windows.