SQL Server 2014 - Reset everything
You should rebuild the system databases.

Setup /QUIET /ACTION=REBUILDDATABASE /INSTANCENAME=InstanceName /SQLSYSADMINACCOUNTS=accounts [ /SAPWD= StrongPassword ] [ /SQLCOLLATION=CollationName]


This would not remove the data and log files relating to the old databases - it just removes all references to them. You would have to manually remove the files to free up the disk space.

You must also install all service packs and hotfixes again.