Access the BIOS and disable the "Native SATA Support" listed there.. XP should then recognize the hard drive and be able to install.. XP has to recognize the hard drive BEFORE you install the SATA drivers anyway.. It's during those initial installation stages where you press the F6 key and install the SATA drivers.
But it's a little confusing since you said: "I'm trying to downgrade a Lenovo N100 to Windows Vista".. But it appears like you're trying to install XP.. So...which operating system do you want to place on the machine.. XP or Vista?
Which SATA controller do you have installed? (You should be able to find it in Device Manager.) The manufacturer of the SATA controller is generally where you will need to get the SATA drivers for the installation.
Hope this helps.
Grif
Hi! I'm trying to downgrade a Lenovo N100 to Windows Vista, but Windows XP doesn't recognize the hard drive Toshiba MK1234GSX. Unfortunately, I have been seeking for these drivers and I haven't been able to find them.
Somebody knows where can I download the SATA drives for this model? I would appreciate very much your help!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic