Yes, I am sorry if you misunderstood - When you go to my computer your drives are labled from a: to z:, however, a: and b: are researved for for floppy / tape drives.
In Windows, the default letter for your hard drive is C:, but it CAN be any other letter too. Every computer is different, and because some computers have several hard drives and several partitions, several CD drives etc it is hard to 'guess' which letter your computer is using.
The chances are pretty good that C: is your hard drive where windows is installed. If you only have one hard drive then D: should be your first CD drive.
For better help, can you please tell us how many hard drives you have.
If you are using partitions on your hard drive.
How many CD drives you are using.
Any other drives you might have such as card readers.
Do you have a C: ? If yes then that should be where windows is installed. If not, then usually the lowest letter (where C: is lower than Z
is your hard drive where windows is installed. But this rule is not solid only default.
Usually when you are in 'my computer' it lists which drives are hard drives, which are CD drives etc. Just look to see if your CD drive is listed. If you have 2 then see if they are both listed.