Hate to say it but you don't have to re-install windows there is a way around that can work but isn't always guaranteed.
But also know that AHCI doesn't really give any "extra" benefits for a desktop.
Source:
How to switch from IDE to AHCI without reinstalling (IP35-pro) - Universal abit USA Forums Code:
I've seen many posts here asking the same question over and over again and i see a lot of misinformation and suggestions and mostly the comment "it does not work".
Well, it does work and it's actually really easy.
1. Since you have Windows XP already installed i expect you also have internet access.
2. Download the Intel matrix storage drivers from intel.
http://downloadcenter.intel.com/Prod...ProductID=1809
3. Open a dos prompt and navigate to the place where you downloaded the driver.
4. type the name of the file followed by -A -A -PC:\<path> (e.g. IATA50_ENU.EXE -A -A -PC:\C:)
5. this extracts the drivers needed on the C:\ drive in 2 folders (for 64 bit and for 32 bit OS'ses).
6. Open up your device manager and navigate to IDE/ATAPI controllers.
7. Find the 2 intel SATA controllers in there and right click one of them and select "update driver". Select to chose a driver and select "have diskette".
8. when asked to specify the location of the driver, browse to the folder you created in step 4 and select the AHCI driver from the list (on the bottom somewhere). It will tell you that the driver is not the right driver for your device but you will simply ignore this warning.
9. Do the same for the other intel SATA controller in your device manager.
10. shutdown the PC, go into the BIOS, change from IDE mode to AHCI and reboot.
11. System should come up and everything is peachy.