Below info found here: http://www.safer-networking.org/en/faq/6.html
Why can't I remove the Sti_Trace.log (or SchedLgU.txt) file? The Sti_Trace.log file is opened on many machines; most often on Windows 2k/XP/ME. The reason is that the Still Image Monitor runs all the time, using this file. You can use msconfig to disable the Still Image Monitor, but as it is of no harm I would suggest to add this log file to the single ignore list.
The same is valid for the SchedLgU.txt; it is the log file of the scheduler. If the scheduler is running, this file is kept open. If you are not using the scheduler, I suggest disabling it, this will not only allow you to back up this file, but also save some RAM.
(To add a problem to the single ignore list, simply right-click on it in the results list, and choose the appropriate menu item.)