If (when) it happens again take a look at your Windows Task Manager to see what is running.
Ctrl-Alt-Del will bring up the Task Manager:
Look at the Applications.
Look at the Processes.
Look at the Performance.
At this time just look. You want to find what is hogging your resources. When, and if you find one process using most of your memory the next step may or may not be a "kill the sumbitch" one.
It could also be a simple thing like a disk which is all scattered to a farethewell and requires a cleanup. Running the Norton stuff, among other things, defragmented your disk and, if it was a mess, would result in a speed pickup.