There are quite many users asked this recently. However, if you upgrade Windows 8 Consumer Preview on other original Windows systems, such as Windows 7, you will have no chance to downgrade back to Windows 7. The only way for you is to format the system drive and install Windows 7 again. In that case, all your settings, and all your data in system driver will be format as well. So please do data backup before you doing so.
But if you installed Windows 8 Consumer Preview in other drive, excluding original system drive, then there is a way for you to get rid of Windows 8 Consumer Preview. Just follow the instructions as bellow:
Stage 1: Remove Windows 8 Consumer Preview from boot loader
You will need a free piece of software called EasyBCD available here.
Once installed, open the software (you will need to be an administrator). Click ‘Edit Boot Menu’ from the sidebar. Then select the ‘Windows 8 Consumer Preview’ and click ‘Delete’. Next tick the checkbox for your previous version of Windows to load that by default. Now click ‘Save Settings’.
Next, from the sidebar, click ‘BCD Backup/Repair’ and select ‘Re-create/repair boot files’ and then click ‘Perform Action’.
You should now have removed Windows 8 Consumer Preview from the boot loader. It is best to check this has worked by restarting your computer before you continue. If your computer makes no mention of Windows 8 Consumer Preview when starting up then you can move onto stage 2. If your computer still boots with Windows 8 Consumer Preview as an option DO NOT move onto stage 2.
Stage 2: Remove Windows 8 Consumer Preview Partition
Open ‘Create and format hard disk partitions’ (search for it in the start menu). Then select and right hand click on the Windows 8 Partition. (Make sure you definitely have the correct partition selected, double check by looking at the side). Click ‘Delete Volume’ and then click ‘Yes’ to continue. Then right hand click on the free space and click ‘Delete Partition’ and then ‘Yes’ to confirm the delete. You should now see the space labelled ‘unallocated’. Select the Windows drive (usually C) and right hand click ‘Extend Volume’. Click ‘Next’, and assuming you want to take up all the unallocated space click ‘Next’ again and finally click ‘Finish’ to complete the operation. You have now completely removed the Windows 8 Consumer Preview.