I am not sure why you cant access your D: drive. The first thing I would do is make sure you have read write privileges on that drive. If you are the administrator or have adimn privileges you should be able to access the D: but maybe they have gotten changed some how. I never used Vista, I went from XP to Win 7 but how you do this has not changed as far as i can tell. In the control panel go to User accounts then/or add remove user accounts. Pick the account that you want to access the drive from and double click it. Then go to change account type  and set to administrator and click change account type. As a standard user you should be able to access the D: but when I was on XP, Windows decided that it wasn't safe for my slave account to access my D: drive I still don't know why. 

User Account video

BEFORE trying the next 2 backup up your PC or if possible burn a system image!!!

If you are a admin your drive may have become corrupt some how if this is the case it can be hard to even get something off of the drive. I have had Great success using Linux based OS like Ubuntu to access corrupt drives but you would have to dual boot Ubuntu to do that. I would call this a last resort, but if you decide that your drive is corrupt this could help you get anything off your drive that you need. There are a lot of videos on you tube on how to dual boot Ubuntu, If you do this make sure you do your research the last thing you want to do is install Ubuntu over windows and lose your OS.

If you do not need any of the data on your D: drive you can whip it and set up a new Partition. Be sure you don't need anything on it before doing this you can not get it back. I am going to give you a link to how to do this but the video shows you how to whip the drive and add the space to your C: drive I don't recommend adding the space to your C: drive, if it is damaged That could be bad. So after you delete the drive and it is black and unallocated right click on the old D: "now black" and reallocate it back to D: drive or what ever letter you want to give it.  

How to delete a Partition