<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def win_find_exe(filename, installsubdir=None, env="ProgramFiles"):
 """Find executable in current dir, system path or given ProgramFiles subdir"""

    for fn in [filename, filename+".exe"]:
        try:
            if installsubdir is None:
                path = _where(fn)
            else:
                path = _where(fn, dirs=[os.path.join(os.environ[env], installsubdir)])
        except IOError:
            path = filename
        else:
            break        
    return path