Toshiba L300 from Vista to XP

Hello,

I have seen people saying
(1) go to bio
(2) go to advanced
(3) choose SATA control mode: from AHCI to compatibility.

It doesn't work for my Toshiba L300. I don't know why.

My CD driver that is embedded into Toshiba Satellite L300 doesn't work,
so I attached Samsung USB External DVD drive.

In this case, which one do I have to use for booting.
I tried DVD/CDROM and USB. Both didn't work, of course, after I changed SATA Control Mode: from AHCI to compatibility mode.

Other said,