Ok, got at a new level today: Installing Intel drivers AFTER M$ Windows XP is installed.
If XP is installed using the IDE compatibility mode for your SATA controller, if you try to change the BIOS to AHCI, XP will turn blue. (Worth to mention that Linux is not complaining if you switch modes back and forth to your wish).
If you want to add the AHCI drivers without reinstalling everything here it is how:
1. Get to the Intel Site and download the latest Intel Matrix Storage Manager Drivers (from here you can get ver. 126.96.36.1993, but look for newer maybe).
2. Unzip it somewhere
3. Get into the command line, get into the unzipped directory and run:
This extracts the driver files but doesn't install them.
4. Go to Device Manager and look for the Intel controller driver. Update it using the "Have disk" option and by indicating the C:\Program Files\Intel\Intel Matrix Storage Manager\winall\Driver directory, where the Intel drivers file where automatically put. Ignore Winows' protests about not being the appropriate drivers but pay attention to choose from the list the driver you need (there are many in there!). Finish the driver update process and reboot.
5.Enter BIOS Setup. Change to AHCI then boot into windows.
6. Once entered in XP with AHCI enabled, install the Intel Drivers fully using the unpacked executable.
7. Reboot again to be sure and double check that in the BIOS you have now AHCI for HDD mode. Then enjoy the speed difference :) (Not much difference people say...)