It must be something peculiar to your install process.
- What is the D: drive? Windows thinks its a network drive.
- What or who created the directory on drive D: named "Items downloaded from browsers to be erased afterwards".
- Do you install calibre 64 bit in a 'non-standard' location, i.e. not in %systemdrive%\Program Files\Calibre2?
- Into which folders do you download, and do you install from there.
- Do you install programs using a 3rd party installer rather than the Windows installer?
- After installing what do you do with the .msi file?
But as theducks said, a registry cleaner should help remove the invalid entries.
BR