Why aren't calculations done using multiple threads?
Because limiting factor in how fast things can go is usually on disk side. Only in situations where same file is processed by multiple algorithms there
could be any expectation of speeding things up and even then thread synchronization would "eat-up" much of benefits.
Do notice that when several files are processed in parallel different processes are used and thus calculations are done on different threads.
Why there are four programs instead of one?
All calculations and right-click processing are done in SummaeExecutor.exe.
Main program (Summae.exe) is alternate shell toward it and allows access to it without need to use command-line.
Sum.exe is essentially same as SummaeExecutor.exe but it uses command-line interface instead of windows and thus it is more appropriate for use in batch files or when interface is not accessible (e.g. Windows Server Core).
Because of Microsoft's User Account Control (UAC) used in Windows Vista and Windows 7, setting right-click associations can only be done by elevated process. SummaeSettings.exe is such process and it is called only when those settings are updated.