Did you UNINSTALL your previous version fully before installing 1.6.12
What board ? What OS ? USB type ports used 2.0 or 3.0 ? these are all very useful bits of information to have in the forum.
I use REVO to uninstall set to max, CCLEANER to get rid of the crud followed by WISE REGISTRY cleaner to root out any stray bits. That method is by far the best way to not only remove programs but to do a thorough clean up of your computer too... both CCLEANER and WISE are advert free and work very well.
There may also be an Arduino folder that gets left if you are on windows.
The path normally used would be
C:\Users\yourname\AppData\Local\ArduinoCreateAgent
or
C:\Users\yourname\AppData\Roaming\ArduinoCreateAgent
I would also recommend saving your SKETCHES folder somewhere else until you have your IDE up and running too.