Dear tnt32,

Recently I faced with the same problem. (I have also a PC with Asus P5K mb.)
I searched google and found this topic at Asus support forum. the topic clearly describes how to crossflash P5K bios with Asus P5KR Mb.'s bios.

Here is the topic:
http://vip.asus.com/forum/view.aspx?...id=1&model=P5K

I tried this method and I could succesfully enable AHCI mode with my P5K mb.

I kindly remind you that it is a risky method and do it with your own risk. (no blame on me)
Also, it is a log thread and you should read till end not to miss any detail.

Good luck.

Regards