drive letter re-assignment on restart

It appears that during a restart the drive letters in the bios settings was swapped.
Previously a scsi drive was c, and an ide drive was d.

Due to the swap, the NTLDR missing message appeared.

The machine is about 3 years old; maybe a cmos battery problem?

I installed a new copy of xp pro on the ide drive and it seems to work ok, and a chkdsk of both disks (fast) indicated nothing serious.

Has anyone seen this particular issue?