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#31
Windows will not allow the Operating System drive letter to be changed.
You must have installed Windows 7 while you were booted into Vista to have got the letter D: as the Windows 7 drive letter.
You see the "System Active" designation, when Windows is designated as "System" Windows will not allow an OS as "System" to be removed or the drive letter changed; that's the reason you could not just delete the Vista partition while it was "System", it's the way Windows 'protects' itself (to be simplistic).
There is no reason not to run Win7 as D drive. But some users don't like it and reinstall anyway.
If you choose to reinstall and already made your Vista Recovery Disks, then I would consider wiping the Vista Recovery partition to recover the space. It can only run from boot now and may not run anyway. If you really want to keep it you can test running it before you wipe the HD to reinstall to see if it works.