As previously mentioned, the most likely cause is that your CGI code is running under default security, which is "Anonymous" access.

You need to either (a)disable anonymous access on the page or directory, from the IIS control (mmc). or
(b)change the user ID inside the script, which can get very complicated.
I recommend (a).

The other thing is that you are using unix-style forward-slashes for the file name. This is usually OK, but for Window's sanity sake, I would suggest:

$file = "\\\\my_machine\\share\\file1.txt";
[download]

The alternative is to use single-quotes (or q()), so you don't have to escape the back-slash.