Help with Truecrypt

I have DVD which should load automatically for me to install a database program, but I get a TrueCrypt error: "Drive letter not available".
My DVD drive letter is D. Can someone please help me.

 

You don't provide a lot of details, but I'll take a guess. It sounds like a DVD containing a TrueCrypt volume was set up in Traveller Mode such that when the DVD is loaded it will automatically run TrueCrypt and prompt you for the password. However, whoever configured Traveller Mode on that DVD specified that the volume must attempt to mount to a specific drive letter, rather than allowing it to mount to the "First available" drive letter. And, the drive letter specified is apparently already in use by your system. For example, D is already in use by your DVD drive. Does this sound approximately right so far?