The docs say that after you get the Apache webserver running, just point your browser to http://localhost/otrs/installer.pl and you'll get a web interface for the database installer. Not so on Windows.
The windows installer puts the installer.pl file in c:\otrs\bin\cgi-bin.
What you can do is edit the httpd.conf file and specify a new ScriptAlias so that your browser will treat .pl files in a specified folder as applications to run, rather than treat them as documents to present as text. Initially it's set to:
ScriptAlias /cgi-bin/ "C:/OTRS/Apache2/cgi-bin/"
that folder has only a printenv.pl in it.
so, add another line after this:
ScriptAlias /otrs/ "c:/OTRS/otrs/bin/cgi-bin/"
If you restart Apache at this point you'll be forbidden access to that directory, so don't forget to explicitly allow access to it in httpd.conf: