To be more exact concerning AHCI mode, Windows will not be able to boot in AHCI mode, unless it was in that state when it was installed. So technically it would be possible to dual boot Ubuntu and Windows after the SATA Operation was changed to AHCI, with out having to switch this mode in the BIOS each time Windows was booted, by reinstalling Windows after the SATA Operation was changed to AHCI. However, this would require a Windows installation disk, and all the required drivers for your system. If your a system builder then you will already have theses requirements. If you are working with a OEM machine then you will not because most OEM’s no longer supply Windows installation disk, opting for a system restore partition instead.