I also get a backup error on a windows server:
Incorrect path or mysqldump utility not executable.
Check $_DB_mysqldump_path definition in config.php.
Variable currently defined as: /usr/bin/mysqldump
I have never tried this backup cause the server is at my office and I do on site backups, but thought I would see what happens
so I changed the path to point directly at the exe file like so:
$_DB_mysqldump_path = 'C:installsmysqlbinmysqldump.exe'
and it worked fine