TrueImage Changed Drives

My neighbour using Trueimage v6 attempted to restore his C: drive from an image he had recently made, and burnt onto a CD (he had also kept the original image on his D: drive) In the process of restoring he received an error message and the restore was aborted.
Removed the Trueimage boot CD he unsuccessfully attempted to reboot into Windows 98 (at this stage he called me for assistance ).
Using a normal boot disk I found that his C: drive was now his D: drive and his D: drive was now showing as his C: drive.
Using the Trueimage boot CD, I restored the image from his original D: drive (now showing as his C: drive) onto the D: drive (actual C: drive) made it active and when the process was completed rebooted successfully.

Any idea what happened, and why this should have happened?