Hi,
I think you only have one system installed on "C" & a recovery boot disk "D".
"D" is a recovery partition containing the "AS NEW" system from purchase for disaster recovery.
Therefore do not delete anything!
What you appear to have done is install Vista on top of Vista, hence the Windows.old files.
To revert back to the original system you will need to uninstall Windows from within the control panel "Programs and files".
Please ensure you have a back-up of your current system before starting any of the above.
if you are in any doubt leave it "As is" because the is nothing wrong with windows!