PC keeps choosing IDE as boot instead of SATA

I don't know if this is a problem with my mobo(ASUS K8V delux) or not but every few weeks my system tries to boot from my 1st IDE drive and not my sata.

I have the latest BIOS for the board and whenever it happens it resets the boot drive to my IDE drive. I then have to go into the bios and select the sata drive as my 1st hard drive and then choose it as the boot drive.

Any ideas why this is happening? Could it just be a prob with the mobo?

possible duff CMOS battery - when it runs low it resets to default BIOS settings which has IDE set as first boot device rather than SATA? Two ways round that - save your correct setup as defaults so that when this happens it doesn't screw things up or replace the CMOS battery. Personally I'd go with the latter.