360 scan will not run -- repeatedly!

So, a while back my 360 scans would not run. Searching these forums, I found a suggestion that I remove and then re-install to fix the problem. I did that, and it worked. 

Then a while later it happened again. Once more, I re-installed. 

We are now on the 4th or 5th time this has happened, and I really would love to not have to keep re-installing Norton every month or so. What could be causing this?