Yes, I can confirm that the un-registered version shows a notification screen on startup to consider registration.
If you do not want it to be displayed on each startup, you can simply disable "load with Windows" option at Configuration -> Preferences page. Then you can be sure that it is not active, it will not display such "nag screen" (and does not monitor the hard disks) and you can manually launch any time manually when required.
If you'd have a full, lifetime registered version (which can be obtained regularly in giveaways if you do not prefer to pay for it) then you can disable the "Display main window when started" at Configuration -> Preferences page. This way Hard Disk Sentinel would really work in the background, show no screen at startup, just monitor and display any problem, issue alerts (if configured) and so.