I hope you have original 'Windows.old' directory inside your primary drive C:
1) Get a boot-able USB for Windows 8 Consumer Preview
[To make a boot-able USB, first format your USB, and then copy the whole content of 'WindowsESD' directory (you would get that inside C: drive after installing windows 8 Consumer Preview) to your USB. Additionally, open C:\WindowsESD\boot\ in command prompt with admin rights and run command 'bootsect /nt60 g:' (without quotes) where 'g:' denotes your USB drive letter. Well, another way to make a bootable USB for Windows 8 is - use 'Windows 7 DVD/USB Download Tool but you must have an ISO image of your "Windows 8 Consumer Preview" installation before hand]]
2) Boot from that boot-able USB device (change your BIOS settings if necessary)
3) Let the setup load
4) Choose languages and click 'Next'
5) You would see a 'Repair your computer' button; click on that
6) Press SHIFT+F10. This would open an elevated command prompt
7) Now, run the following commands to move your existing Windows 8 installation:
[Ignore if you get 'no such file or directory exists' or something like that and go the next step]
- md win8
- move Windows win8\Windows
- move "Program Files" "win8\Program Files"
- move "Program Files (x86)" "win8\Program Files (x86)"
- move Users win8\Users
- attrib -h -s -r ProgramData
- move ProgramData win8\ProgramData
- move PerfLogs win8\PerfLogs
- rd "Documents and Settings"
8) Now, restore your previous Windows installation (for both 32bit and 64bit) with the following commands [Ignore if you get 'no such file or directory exists' or something like that and go the next step]
- move /y c:\Windows.old\Windows c:\
- move /y "c:\Windows.old\Program Files" c:\
- move /y "Windows.old\Program Files (x86)" c:\
- move /y Windows.old\Users c:\
- move /y Windows.old\ProgramData c:\
- move /y Windows.old\PerfLogs c:\
- move /y "c:\Windows.old\Documents and Settings" c:\
** If your previous installation was WinXP SP3, run the following command additionally:
- g: (again, 'g:' denotes your USB drive letter)
- cd boot
- bootsect /nt52 c:
9) Now, reboot your computer and boot from your hard disk