I had a request from a company here in South Africa to downgrade a new Dell Inspiron 1720. It was not able to find a hard disk during install but I found a nice article.

In BIOS, under OnBoard Devices:

1. Modify the Flush Cache Module to say "Disabled"
2. Modify the SATA Operation from "AHCI" to "ATA"
3. Reboot with your XP CD.