What is the best way to find out where notepad.exe and mspaint.exe are that will work across various versions of Windows?
Should I get the Windows directory via SHGetFol
I think to start off small you should get the windir environment variable and look in the subfolders %windir%\system32\ for mspaint and notepad. Most likely they will be there.
windir
%windir%\system32\
mspaint
notepad
However if that fails, well then resort to a more brute force search.