IRST is also an AHCI driver, and works fine in AHCI mode.
To replace it with msahci, go into Device Manager from Control Panel. You'll need to find the entry for your SATA controller, either under Storage Controllers or IDE/ATAPI, it will say Intel - something, since I don't know what mother board you have I can't say exactly. Double click that to open its Properties. Click the Driver tab, it should say Intel and the version number. Click the Update driver button, and then "Let me pick from a list...". Select that and you should find an entry for "Standard AHCI SATA Controller...", which is msahci. Select it and follow the prompts to finish. You MUST reboot the PC to complete the process.
After the reboot go into Device Manager again, and find the SATA controller again, it should be in the IDE/ATAPI category, and check the Properties again to be sure msahci is now the driver. If so, close that, and uninstall IRST from Programs and Features.
