I got the same MB (Asus P5QL-E, intel P43/ICH10R) and followed all the steps.
IaStor.sys is in "C:\WINDOWS\system32\drivers"
iastor-ahci+raid.reg added to registry
Checked for the reg keys to be in their place (for AHCI - that's what i need to enable)
Rebooted in AHCI mode (Windows XP pro sp3 was installed in IDE mode).
It behaves as if i had done nothing. Bluescreen and reboot.
Did you do anything else but mizaring's guide to make windows boot in AHCI on Asus P5QL-E?
Hope to get an answer soon. Thank you all in advance!
Originally Posted by Notme 
@mizaring
Thanks for the update for Intel ICH10 and I tested it on a new Asus M/B P5QL-E (Intel P43) with Seagate ST3500320AS for XP Pro SP3, it works!!
For those who want to know how to enable hard drive NCQ with AHCI which was the reason for me to use AHCI in the first place, according to the information on Seagate's web site, it is by default enabled, if you are using AHCI and it can not be changed by end user level as it is on the firmware level (for those HDDs support NCQ).
Thanks and hope this will help others too.