|mm perhaps also...; so as to ensure you are able (still) to boot to version you wish to retain...|
Boot the version you wish to keep; presumably on c: drive?
Set it as default OS to boot; then reboot - to verify it does just that...
Then delete the version on d: drive; and then edit the boot.ini so as to delete the entry there for the version on d: drive...
To set default OS to boot... and also easily edit the boot.ini go via:
MyComputer\Properties\Advanced\startup/recovery - settings
When you edit the boot.ini.... cafrefully note the entry for the default OS to boot; and ensure you do NOT delete that (main) entry in the main body of the boot.ini below that default OS line... The one you will delete will be slightly different to the one for the default OS on c: drive (the version you wish to retain).