Right Click on My Computer and choose Manage.
Then click on Disk Management.
You will see a list of your drives in the bottom pane. By right clicking on the right hand section of the particular drive you can choose to change the drive letter.
If you have loaded software that then requires the CD to be present you will have to reload if you change the CD drive letter from D to E - as the installed softwrae will still look for the disk on D