Generally speaking, you can make changes to your DVD drive letter in two ways.
The first is by changing the MBR of your disk. To do this follow these steps:
1) Open Disk Utility on Mac OSX or "Disk Management" on Windows
2) Select the hard drive/disk that contains windows/OS X.
3) Select the partition that holds windows, usually C, and edit it (i.e change the type if necessary). Restart the computer while holding the CTRL key to enter Safe Mode with Command Prompt. Some versions may require F8 or some other key to select boot menu options --- use Google for help with this step if needed...