just had this happen; letter changed from G to E. also added a few from repeated plugin of memory stick and CF card readers. used the above method to check drive names and found the had not been changed.. absolutely no idea as to what actually happened..shut the computer down for a day and everything reset to normal...
Exactly what happened to you, a flash card or usb drive bumped your cd up a lettr, the name won't change just the drive letter. A reboot without those other externals let the drive get back its original letter.
Mike's advice to assign your optical drive a high letter is the best option but you need to do it before you install anything from a disk or when Office wants you to insert the disk it won't find it. I always used "S:" for see dee and did that from the real mode drivers on my boot floppy before installing Windows, those days are long gone now.
Floppy ? ? ? ... LOL