What do you mean by "open" them? Make them appear in a viewer or video player? And when you say "open" a folder, do you mean open it in Windows Explorer?
To "open" a file in its default program, type its file name at the prompt and press ENTER.
To open a folder in Windows Explorer type
Start "" "c:\path\to\folder" (replace with the right path)
To open the current directory type
Start "" "%cd%"