I copy your scripts

OS: Windows XP sp2

Script:
Break on
$path = %path%
$path = $path + ";c:\a"
?$path
Sleep 2
Set "path = $path"
?@ERROR
?@SERROR
Sleep 2

In c:\a I have a executable banner.exe
START, run , execute banner.exe and fail
I just ran the script the path variable not have c:\a