Some settings are changing spontaneously

I have always had cookie scanning set to ignore.  Now, for the third time in several weeks, I find the setting on this pc has spontaneously reverted to the default of "ask."  My setting to not do any idle scans has also changed by itself, to weekly (which I believe is the default).

 

I haven't checked the other settings in detail, but don't notice any other changes.

 

What could be causing this?  I haven't noticed any such spontaneous changes on my other NIS 2010 pc (where I also choose ignore cookies, but DO have it set to do weekly idle scans).  Both pc's are version 17.5.0.127, Winx XP Home SP2.

 

Thanks.