The scheduled task (Admin CP > Scheduled Task > POP3/IMAP fetching) setup in the help desk is called 'pseudo cron'. Pseudo cron mechanism has been provided as a fall-back mechanism for the clients, who are yet to configure cron jobs on their servers. It is not fully reliable since it works only if an active staff is logged in the Staff CP. The reliable way to go about this would be to get a cron job (scheduled task in Windows environment) configured for the same purpose through either the web control panel or shell access.
Jitender Kumar