Hi,
Thank you for posting your question in Microsoft Community.
From your issue description, I understand that **you are unable to change Windows XP to use AHCI disk mode rather than IDE mode for Dual boot.**Please let me know if this is not the issue that you are facing.
Before proceed with troubleshooting, I need some information.
What is the make and model number of the computer?
Try the steps below and then check:
Step 1: Update the driver:
a) Plug SSD drive.
b) Click Start button, click Run.
c) In the Run window, type devmgmt.msc and then click “Ok”.
d) In the Device Manager, expand “IDE ATA/ATAPI controller”.
e) Under the list, check if any listed hardware is titled as “Standard Dual Channel PCI IDE Controller or SATA IDE Controller”. If the device is listed as the same, right click on it and select “Update Driver”. Follow the onscreen instructions to update the driver.
f) Note: If you find other than the list provided as above, please post your reply with the name of drives listed under “Intel ICH9 SATA IDE controller”.
Step 2: Edit the BIOS and then modify the settings:
BIOS Disclaimer: Modifying Basic Input Output system (BIOS)/ complementary metal oxide semiconductor (CMOS) settings incorrectly can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the configuring of BIOS/CMOS settings can be solved. Modifications of the settings are at your own risk.
Note: You can also contact the manufacturer of the system for the same.
If the issue persists, please feel free to reply to us with the results and questions asked as above. We are happy to assist you further!
Hope it helps.