Background tasks stop PC responding

My Windows7 i5 laptop has had NIS since new and I've recently upgraded to the latest version.

But it has always had the problem that after leaving the machine idle for a short time, it stops responding.

 

I think this problem occurs when I've installed some new software, but I'm not 100% sure.

I have switched off the idle-time optimiser and that makes no difference. 

 

When it happens, I see a box at the bottom right of the screen saying "Norton Internet Security is currently performing background tasks while your computer is idle".  I can click the x on that box to hide it, but then nothing else responds.  I have to power it off by holding down the power key.

 

I have seen that some people have been able to fix this by running the Norton uninstall and re-install utility but I tried that and it made no difference.

 

I have just checked in the history and there aren't any Norton tasks showing as running in the time when the machine hung, although there was a message about background tasks running.

 

I'm generally very happy with NIS but if I can't resolve this I'll be changing to another product.

 

Any help much appreciated.