Tracking this one down can sometimes be difficult.
A couple of things you said are "disturbing".
You said '...highlighting the "Left$"'.
Is the "$" part of the code? I don't recognize that
convention. I've used it other places referring to addresses, but
not with functions. Could it be that you intended to use
the "left(" function  but somethow got a $ inserted and it's trying
to find Left$ in the function library?
"... a couple of files  with this code  now say..."
not ALL files with this code?
In the VBA editor of the 2003 application, check
the References panel and see if there are any "not found" libraries.
sometimes, if there is a library that cannot be found, it
fails to load the rest of the libraries.
Can you post a snippet of the offending code?