I assume you are using Norton 360 anti virus?
For some reason this app quarantines (removes) the WorkingHours.exe file because it thinks it would be harmful (it is not).
I already reported this false positive to Norton twice - without any response.
(just another reason for me personally not to touch any antivirus program any more, they just cause more problems than they promise to solve IMHO)
To fix the problem, you need to tell Norton to "un-quarantine" the file or whitelist WorkingHours. I don't have Norton so I can't tell you where in the UI you would find those options