Missing Desktop Due To MBR?

Some months back I installed Vista 32-bit Home Premium in C: partition. When W7 came out, I installed W7 Ultimate 64-bit in D: as a dual boot. Now I want to keep Vista, but use W7 as the main OS I am going to build on from here on out, so I want it at the front of the hard disk in C: (where it will run faster) and my secondary or experimental OS in D: (Vista).

So I backed up Vista to an external drive, booted from a CD (DriveImage XML BartPE) and restored the Vista image to D: ... then reinstalled W7 in C:.

When I booted after re-installing W7, the multi-boot menu listed Windows 7 twice, instead of Windows 7 and Vista, which I kind of expected, because W7 *had* been in D: and now it was in C: "too." I used Easy BCDEdit to correct the text.

Now the machine boots into W7 fine, but when I try to boot to Vista, I get the usual logo, then the Welcome screen, which lasts a bit longer than it would if nothing was hooey... then I get a cursor (responsive) and a black (empty) desktop.

I used Ctl Atl Del to access Task Mgr, and tried clicking the New Task button and entering explorer.exe, but it is looking at my C: drive for explorer, rather than where it SHOULD be looking in D:, because a funky, unstable, nonfunctional W7 desktop finally came up INSIDE VISTA!! (mind you, W7 is 64-bit!)

I shut down and tried booting into safe mode.... Got the same result of a black screen, and tried entering D:windowsexplorer.exe but got the same error message:

"...ieframe.dll is either not designed to run on Windows or it contains an error..."

..and again the W7 desktop "tried" to come up.

I suspect it's looking in W7's folder (C:) because the backup image I made of Vista probably contains the original MBR, which mapped Vista to C:. I was hoping when I re-installed W7 to C: with the restored VISTA image already in D: that W7 would create a correct MBR... and it did, I think, but it left the old MBR at the beginning of the D:drive TOO... so when I boot to D:, the MBR record [ON D:] is telling VISTA it is in C:!! Hence Vista goes pokin around in my W7 (C:|) directory.

I downloaded the bootable Vista Recovery CD but it didn't find any problems, and I believe that's because VISTA isn't the problem... the MBR on D: is!

I don't want to have to rebuild the Vista OS... just need to remove the incorrect (old) MBR from the beginning of the D: drive... Can I do that?? Can I test this theory by somehow reading the first sector of that drive?