[DNSB]

I'm going to assume that you are installed the installed calibre not the portable calibre. To change the install directory, open the calibre installer. On the first page where you see the accept the license terms, check that box and then click on the Advanced button. Change the install directory to D:\Program Files\Calibre2\ (you may have to create the Program Files directory on your D: drive first). Click next and then click install. See the attached images.

If your previous install on your D: drive was the calibre portable installer, this is unlikely to carry over all your settings but will leave that for the calibre gurus to answer.

EDIT: Tested this installing on a Windows VM and once I installed to the D: drive, the next install automatically defaulted to that drive so that suggests that you installed the portable version on your D: drive.