SQL Server Management Studio 2018 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\18.0\UserSettings.xml
SQL Server Management Studio 2016 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\13.0\SqlStudio.bin
SQL Server Management Studio 2014 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\12.0\SqlStudio.bin
SQL Server Management Studio 2012 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\11.0\SqlStudio.bin
SQL Server Management Studio 2008 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin
SQL Server Management Studio 2005 delete the file – same as above answer but the Vista path. C:\Users\%username%\AppData\Roaming\Microsoft\Microsoft SQL Server\90\Tools\Shell\mru.dat
These are profile paths for Vista / 7 / 8 /10.