1) ShellExecute with open mode will use default program associated (the kind which will be launched if you double-click on file)
2) Windows kernel works with forward slashes perfectly starting with at least WinNT. So use them instead of backward ones which needs escaping. (Or at least use raw string literals).
Literally only place which does not support forward slashes properly is cmd console: it threats them as additional arguments unless whole line is wrapped in quotes (PowerShell works fine)
//Does not work
C:\Users\MiiNiPaa>dir C:/users
//Works fine
C:\Users\MiiNiPaa>dir "C:/users"