Most of the embedded systems I design output data in a CSV format for performance analysis, monitoring and debugging. So I add LibreOffice Calc to the CSV files Open with Explorer option for convenience. When moving from OpenOffice, or upgrading LibreOffice versions, the file association is sometimes broken do to left over registry keys pointing to the wrong place for scalc.exe.
In the past I would manually delete the left over entries using RegEdit, a rather slow and boring task. This post on the OO forum points out the easiest method of fixing this issue, using CCleaner. I like to use the portable version because I rarely run the program so by the time I want to run it again I need to get the latest version anyway.