Windows 8 release preview uninstallation problem

I installed windows 8 RP on my desktop in july 2012 from digit dvd.
but its trial period expired within a month. hence it was of no use to me.

but i dont find any option to uninstall it in the system.

Also, i have overwritten it ( Win8 RP) on windows 7. can i downgrade it to win7 now..? if not how can i remove it completely

without formatting the drive and keeping my data intact on same drive..?