Doh, where are my manners? Welcome to the Seven forums!
You might want to leave task manager open/minimized with the icon showing in down by the clock (system tray, notification area, whatever it is called this time around :-)
That way, you can glance at the CPU percentage when things bog down.
If you want - you can replace Norton 350 with MSE
just for a day or two and see how things go.