Unfortunately some programs will clash with the Timer Software when it is operating. Depends on how powerfull the computer and the type of operating system. You may have to shutdown firewalls, anti-viruses or program schedulers such as scandisk or defragmenters one at a time to see which is the problem.
Screen savers can cause a computer to lock up also.
If you are using a stand-alone computer, that is not used for any other purpose, you won't need any extra programs running at the same time. The monitor can be left off when the timing settings are not being altered.