You can change the drive letters very simply under Control Panel, Adminstrative Tools, Computer Management, Disk management. Very easy to do.
You cannot change the C drive, as it your boot disk. If your "C" drive did not end being the one you wanted it to be, you are best to just start all over.
To keep this from happening, when you install a new OS, only hook up 1 HDD and your DVD/CD drive. Then after the install attach your secondary drives.
Be warned that if you have installed programs to the drive that is now labed as "D", and you change it to, I am assuming you want it to be "E", and your DVD/CD to be D, you may have problems with the programs you have installed, as they will be keyed to, or think they are still on the "D" drive.
Likewise, any programs you have installed on C or D either one by using the DVD/CD drive, like games, when you insert the disk to play to them, will try to look back to drive "E" to find the CD or DVD, and they will give you an error message that they cannot find the CD/DVD.
If you haven't installed anything since the OS load though, you can simply follow the instructions above and change the 2 drive letters, no big deal.