@Maria Loranger - something seems to be wrong with permissions, did you reboot after uninstalling calibre 64 bit, cleaning registry etc?
You could try this:
- Close calibre
- Make sure you are logged in with admin privileges
- Delete this folder ==>> C:\ProgramData\Microsoft\Windows\Start Menu\Programs\calibre 64bit - E-book Management\
Now try installing calibre 64 bit, if it works you can uninstall the 32bit version.
BR