Windows 8 not booting up properly

Help Needed
I installed windows 8
but after few days it started taking forever to load
furthermore i didnt like it too much
so when i formatted it and installed windows 7
it installed ok but at the end of installation it gave
"logon personalization failed" error and when i tried booting from dvd an error comes which does not load win 7
now i have installed back win 8
but it is giving error failed \boot\bcd
can anyone tell me wats this error
and how to fix it