In a recent upgrade, I replaced a couple of older, smaller hard disks (one SATA, one parallel ATA) with a new 500 GB SATA disk. I retained another old PATA drive. So the new setup has one (new) SATA and one (old) PATA.

The BIOS of my Asus M2N-MX mobo is set to boot from the SATA drive. But, since this upgrade, I find that every time I install new software the boot order changes and the PC tries to boot from the PATA drive. Since this drive is not bootable, I get the "NTLDR is missing" message. I reset the BIOS boot order and everything is OK again until the next time I install new software - and this can be as little as the recent upgrade to Spybot, or Windows updates.

Any suggestions as to how to fix this?