Use ShellExecute (API function)

Ex: DECLARE INTEGER ShellExecute IN SHELL32.DLL INTEGER p1, STRING p2, STRING p3, STRING p4, STRING p5, INTEGER p6
p1=0
p2='Open'
p3='c:\notepad.txt'
p4=''
p5=''
p6=1

lnReturn=ShellExecute(p1,p2,p3,p4,p5,p6)

Be sure the file exists. In case of error, lnReturn helps too.
To have notepad window maximized or minimized or other facilities-play with the parameters- you may need to read the MSDN documentation for that too.