Calibre makes very littl use of the Registry and it also leaves the Library and settings intact.
There is a (Windows, normally) hidden folder:
C:\Documents and Settings\
your_user_log_on_name_here\Application Data\calibre.
The folder is also reachable (when Calibre is running) from Preference: Miscellaneous: Open Configuration Folder.
Deleting the folder before re-installing, cause ALL Calibre settings to be forgotten.
The Library is intact,
You will need to point the new installation towards the existing library.
Download a Fresh copy of Calibre from the Official site. Do NOT use a
Download Manager program to do so.