Alright, I just got my ACHI working without re-installing XP or crashing! I have an ICH8R southbridge though, so whether or not it can help here, hard saying. I found too many instructions to fail when I tried them. So I found a guy who did this to his laptop and it worked for me...I'm sure instructions given are the same or very similar...I'm just posting what worked for me.
1- download Intel Matrix Storage Manager exe.
2- Put in C:\ Dir
3- Go to Start, Run, type c:\iataxxx.exe -a
This will extract the files to C:\Program Files\Intel\Intel Matrix Storage Manager
4- Go to Control Panel > System > Hardware > Device Manager
Then go to IDE ATA/ATAPI Controllers, right click one of the first Intel PCI Storage 802xx controllers, not the primary or secondary below them, Click Update Driver, manually find the drivers, don't search, then click Have Disk,
Copy and Paste:
C:\Program Files\Intel\Intel Matrix Storage Manager\Driver
Then locate 82801HR/HH/HO SATA ACHI Controller (very bottom one, at least for ICH8R)
It will come up with a warning, click okay, it will ask to restart, do so...and go into Bios, set your SATA to ACHI and voila, you should be ACHI. This took all of 5 minutes from download to completion, without digging into the case, no floppy/cd, all good to go. Again, sorry if it does not help, I was stoked I could pull it off in XP since it is easy in Vista...pretty much the same deal in XP I just realised. Dunno why there are so many sites with more complicated instructions (even for ICHxR's) if it's this easy.