Check these keys for Intel chipset, set to 0

>HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Msahci\Start  
>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\IastorV\Start

Changing from RAID to AHCI, you may need one more step. Msahci driver requires Atapi.sys miniport to be loaded during boot as well.

>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\atapi\Start

For AMD chipsets

>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\nvstor\Start  
>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\nvraid\Start  
>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\amdsata\Start (non raid)  

If You are not able to boot up thanks to ``BSOD``, try out booting to ``Safe Mode`` and setting those Registry keys.