This was my scenario when 'downgrading' an Inspiron 1525 from Vista to XP Pro:
I had to try the 'easy way', since the hard way failed for me. All appears to be fine now, but of course i'm left in ATA mode, when i'd like to be in AHCI mode. I tried to install the correct SATA drivers, but when switching back to AHCI, i get a very early BSOD and an endless reboot cycle. Certainly the aforementioned
"if you switch to AHCI in the future windows will crash."
is the case. But i was hoping for a different result since the correct drivers are theoretically in the system. I'm hoping i haven't got a bootstrapping problem here, in that in order to install the drivers properly, ahci needs to be enabled and i need to be in Windows ...