The Tweakers Asylum have just finished putting up a Windows XP ACPI Tweak Guide. By disabling ACPI under Windows XP they saw frame rate increases of up to 16 FPS in Quake 3. Here are the details:

    "Recently I received a rather excited ICQ message from Tempest, our Windows 2000/XP nut. "I know how to get higher frames per second in Windows XP!" Of course this peaked my interest so I bit, "How's that?" To my surprise Tempest replied with, "I disabled ACPI in Windows XP and gained 14 FPS in Quake 3!"

    I must admit at first I was somewhat skeptical so I decided to have both IRONCHEF (Intel P4 System) and I (AMD TBird System) conduct separate evaluations of Tempest's claims."