Exchange of C: Partition to D: Strange thing!

Hi! Everyone, i just formated and reinstalled win XP Service pack 3. Like always i partition my drive into C for the system files and all the programs I’m running, and drive D for my very important staff.
When ever I’m doing a new Windows installation i never delete the D drive, i only format the C drive and proceed with the installation on this drive, now here is the strange thing “last night after finishing the window installation i realized that my D drive became the C drive and the C drive is the D drive and thats where the OS is” Well I’m surprised never seen this before and at the same time I’m worried with my files being on the main drive C: instead of on the D: drive. Guys what is the cause of this behavior and is this normal? How can stop this from happening again next time? Thanks in advance for your attention.